[llvm] [InstCombine] Fix FMF propagation in `copysign Mag, (copysign ?, X) -> copysign Mag, X` (PR #121574)

[Matt Arsenault via llvm-commits]

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@@ -2645,8 +2645,12 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     // copysign Mag, (copysign ?, X) --> copysign Mag, X
     Value *X;
     if (match(Sign, m_Intrinsic<Intrinsic::copysign>(m_Value(), m_Value(X)))) {
-      II->andIRFlags(Sign);
-      return replaceOperand(*II, 1, X);
+      IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
+      Builder.setFastMathFlags(II->getFastMathFlags() &
+                               cast<Instruction>(Sign)->getFastMathFlags());
+      Value *CopySign =
+          Builder.CreateBinaryIntrinsic(Intrinsic::copysign, Mag, X);
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arsenm wrote:

We really ought to add FMF arguments to all these CreateFunctions, the guard + set is so clumsy 

https://github.com/llvm/llvm-project/pull/121574