Handle no-postdominator case in finalizer pass (#54765)

This pass was assuming that the post-dominator of all finalizer uses
exists as a real BB in the CFG.

Resolves #54596

base/compiler/ssair/domtree.jl

@@ -657,6 +657,8 @@ end
 Compute the nearest common (post-)dominator of `a` and `b`.
 """
 function nearest_common_dominator(domtree::GenericDomTree, a::BBNumber, b::BBNumber)
+    a == 0 && return a
+    b == 0 && return b
     alevel = domtree.nodes[a].level
     blevel = domtree.nodes[b].level
     # W.l.g. assume blevel <= alevel

base/compiler/ssair/passes.jl

@@ -1614,6 +1614,7 @@ function try_resolve_finalizer!(ir::IRCode, idx::Int, finalizer_idx::Int, defuse
     end
     all(check_defuse, defuse.uses) || return nothing
     all(check_defuse, defuse.defs) || return nothing
+    bb_insert_block != 0 || return nothing # verify post-dominator of all uses exists
 
     # Check #3
     dominates(domtree, finalizer_bb, bb_insert_block) || return nothing

test/compiler/irpasses.jl

@@ -1949,3 +1949,24 @@ let code = Any[
     # fallthrough
     @test isdefined(ir[SSAValue(gotoifnot+1)][:inst]::ReturnNode, :val)
 end
+
+# https://github.com/JuliaLang/julia/issues/54596
+# finalized object's uses have no postdominator
+let f = (x)->nothing, mi = Base.method_instance(f, (Base.RefValue{Nothing},)), code = Any[
+   # Basic Block 1
+   Expr(:new, Base.RefValue{Nothing}, nothing)
+   Expr(:call, Core.finalizer, f, SSAValue(1), true, mi)
+   GotoIfNot(false, 6)
+   # Basic Block 2
+   Expr(:call, Base.getfield, SSAValue(1), :x)
+   ReturnNode(SSAValue(4))
+   # Basic Block 3
+   Expr(:call, Base.getfield, SSAValue(1), :x)
+   ReturnNode(SSAValue(6))
+]
+   ir = make_ircode(code; ssavaluetypes=Any[Base.RefValue{Nothing}, Nothing, Any, Nothing, Any, Nothing, Any])
+   inlining = Core.Compiler.InliningState(Core.Compiler.NativeInterpreter())
+   Core.Compiler.verify_ir(ir)
+   ir = Core.Compiler.sroa_pass!(ir, inlining)
+   Core.Compiler.verify_ir(ir)
+end