JIT: refactor logic for removing box upstream side effects (#13169)

Introduce a new method `gtTryRemoveBoxUpstreamEffects` to capture
the logic for cleaning up after a dead value type box into a utility.
Call this from `gtFoldExprSpecial` and now also from the inliner when
it finds a dead box argument.

Also remove the useless `fgExpandInline` field and associated
`fgIsInlining` method, as the jit never set the field to true.

No diffs from the refactoring. The inliner case kicks in in 3 places
in the test tree.

Closes #13136.

src/jit/compiler.h

@@ -2228,6 +2228,7 @@ class Compiler
         gtFoldExprConst(GenTreePtr tree);
     GenTreePtr gtFoldExprSpecial(GenTreePtr tree);
     GenTreePtr gtFoldExprCompare(GenTreePtr tree);
+    bool gtTryRemoveBoxUpstreamEffects(GenTreePtr tree);
 
     //-------------------------------------------------------------------------
     // Get the handle, if any.
@@ -3614,9 +3615,8 @@ class Compiler
     bool fgFuncletsCreated; // true if the funclet creation phase has been run
 #endif                      // FEATURE_EH_FUNCLETS
 
-    bool fgGlobalMorph;  // indicates if we are during the global morphing phase
-                         // since fgMorphTree can be called from several places
-    bool fgExpandInline; // indicates that we are creating tree for the inliner
+    bool fgGlobalMorph; // indicates if we are during the global morphing phase
+                        // since fgMorphTree can be called from several places
 
     bool     impBoxTempInUse; // the temp below is valid and available
     unsigned impBoxTemp;      // a temporary that is used for boxing
@@ -4479,12 +4479,6 @@ class Compiler
 
     static GenTreePtr fgGetFirstNode(GenTreePtr tree);
     static bool fgTreeIsInStmt(GenTree* tree, GenTreeStmt* stmt);
-
-    inline bool fgIsInlining()
-    {
-        return fgExpandInline;
-    }
-
     void fgTraverseRPO();
 
     //--------------------- Walking the trees in the IR -----------------------

src/jit/flowgraph.cpp

@@ -90,9 +90,8 @@ void Compiler::fgInit()
     genReturnBB = nullptr;
 
     /* We haven't reached the global morphing phase */
-    fgGlobalMorph  = false;
-    fgExpandInline = false;
-    fgModified     = false;
+    fgGlobalMorph = false;
+    fgModified    = false;
 
 #ifdef DEBUG
     fgSafeBasicBlockCreation = true;
@@ -22676,6 +22675,12 @@ GenTreePtr Compiler::fgInlinePrependStatements(InlineInfo* inlineInfo)
                     }
 #endif // DEBUG
                 }
+                else if (argNode->IsBoxedValue())
+                {
+                    // Try to clean up any unnecessary boxing side effects
+                    // since the box itself will be ignored.
+                    gtTryRemoveBoxUpstreamEffects(argNode);
+                }
             }
         }
     }