[llvm] [InstCombine] Fix the correctness of missing check reassoc attribute (PR #71277)

[Matt Arsenault via llvm-commits]

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@@ -607,176 +781,9 @@ Instruction *InstCombinerImpl::visitFMul(BinaryOperator &I) {
   if (Value *V = SimplifySelectsFeedingBinaryOp(I, Op0, Op1))
     return replaceInstUsesWith(I, V);
 
-  if (I.hasAllowReassoc()) {
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arsenm wrote:

Can you pre-commit moving all of this code to a helper function in an NFC commit? It's hard to see the real diff here 

https://github.com/llvm/llvm-project/pull/71277