JIT: one phi arg per pred

Revise SSA to add one phi arg per pred instead of one phi arg per ssa def.
This unlocks some cases for redundant branch opts.

In some pathological cases we may see extremely long phi arg lists. Will
keep an eye out for that and perhaps modify the strategy if we end up
with hundreds or thousands of phi args.

Addresses an issue noted in dotnet#48115.

src/coreclr/jit/ssabuilder.cpp

@@ -551,7 +551,8 @@ void SsaBuilder::InsertPhi(BasicBlock* block, unsigned lclNum)
 }
 
 //------------------------------------------------------------------------
-// AddPhiArg: Add a new GT_PHI_ARG node to an existing GT_PHI node.
+// AddPhiArg: Ensure an existing GT_PHI node contains an appropriate PhiArg
+//    for an ssa def arriving via pred
 //
 // Arguments:
 //    block  - The block that contains the statement
@@ -563,14 +564,32 @@ void SsaBuilder::InsertPhi(BasicBlock* block, unsigned lclNum)
 void SsaBuilder::AddPhiArg(
     BasicBlock* block, Statement* stmt, GenTreePhi* phi, unsigned lclNum, unsigned ssaNum, BasicBlock* pred)
 {
-#ifdef DEBUG
-    // Make sure it isn't already present: we should only add each definition once.
+    // If there's already a phi arg for this pred, it had better have
+    // matching ssaNum, unless this block is a handler entry.
+    //
+    const bool isHandlerEntry = m_pCompiler->bbIsHandlerBeg(block);
+
     for (GenTreePhi::Use& use : phi->Uses())
     {
-        assert(use.GetNode()->AsPhiArg()->GetSsaNum() != ssaNum);
+        GenTreePhiArg* const phiArg = use.GetNode()->AsPhiArg();
+
+        if (phiArg->gtPredBB == pred)
+        {
+            if (phiArg->GetSsaNum() == ssaNum)
+            {
+                // We already have this (pred, ssaNum) phiArg
+                return;
+            }
+
+            // Add another ssaNum for this pred?
+            // Should only be possible at handler entries.
+            //
+            noway_assert(isHandlerEntry);
+        }
     }
-#endif // DEBUG
 
+    // Didn't find a match, add a new phi arg
+    //
     var_types type = m_pCompiler->lvaGetDesc(lclNum)->TypeGet();
 
     GenTree* phiArg = new (m_pCompiler, GT_PHI_ARG) GenTreePhiArg(type, lclNum, ssaNum, pred);
@@ -1186,29 +1205,12 @@ void SsaBuilder::AddPhiArgsToSuccessors(BasicBlock* block)
                 break;
             }
 
-            GenTree*    tree = stmt->GetRootNode();
-            GenTreePhi* phi  = tree->gtGetOp2()->AsPhi();
-
-            unsigned lclNum = tree->AsOp()->gtOp1->AsLclVar()->GetLclNum();
-            unsigned ssaNum = m_renameStack.Top(lclNum);
-            // Search the arglist for an existing definition for ssaNum.
-            // (Can we assert that its the head of the list?  This should only happen when we add
-            // during renaming for a definition that occurs within a try, and then that's the last
-            // value of the var within that basic block.)
+            GenTree*    tree   = stmt->GetRootNode();
+            GenTreePhi* phi    = tree->gtGetOp2()->AsPhi();
+            unsigned    lclNum = tree->AsOp()->gtOp1->AsLclVar()->GetLclNum();
+            unsigned    ssaNum = m_renameStack.Top(lclNum);
 
-            bool found = false;
-            for (GenTreePhi::Use& use : phi->Uses())
-            {
-                if (use.GetNode()->AsPhiArg()->GetSsaNum() == ssaNum)
-                {
-                    found = true;
-                    break;
-                }
-            }
-            if (!found)
-            {
-                AddPhiArg(succ, stmt, phi, lclNum, ssaNum, block);
-            }
+            AddPhiArg(succ, stmt, phi, lclNum, ssaNum, block);
         }
 
         // Now handle memory.
@@ -1333,24 +1335,10 @@ void SsaBuilder::AddPhiArgsToSuccessors(BasicBlock* block)
                         continue;
                     }
 
-                    GenTreePhi* phi = tree->gtGetOp2()->AsPhi();
-
-                    unsigned ssaNum = m_renameStack.Top(lclNum);
+                    GenTreePhi* phi    = tree->gtGetOp2()->AsPhi();
+                    unsigned    ssaNum = m_renameStack.Top(lclNum);
 
-                    // See if this ssaNum is already an arg to the phi.
-                    bool alreadyArg = false;
-                    for (GenTreePhi::Use& use : phi->Uses())
-                    {
-                        if (use.GetNode()->AsPhiArg()->GetSsaNum() == ssaNum)
-                        {
-                            alreadyArg = true;
-                            break;
-                        }
-                    }
-                    if (!alreadyArg)
-                    {
-                        AddPhiArg(handlerStart, stmt, phi, lclNum, ssaNum, block);
-                    }
+                    AddPhiArg(handlerStart, stmt, phi, lclNum, ssaNum, block);
                 }
 
                 // Now handle memory.