[llvm] [InstCombine] Fold more 'fcmp' 'select' instrs idioms into 'fabs' (PR #83381)

[Yashwant Singh via llvm-commits]

yashssh wrote:

Thanks for the example @andykaylor!

I tried a change that sets `fast` flag on `phi` nodes created by SROA 
```
diff --git a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
index 88b05aab8db4..1d85c7e0d8a6 100644
--- a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
+++ b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
@@ -1056,6 +1056,22 @@ bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
   // BasicBlock.
   PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB),
                        Allocas[AllocaNo]->getName() + "." + Twine(Version++));
+
+  if(BB->getParent()->hasFnAttribute("unsafe-fp-math")){
+    PN->setFast(true);
+  }
+
   PN->insertBefore(BB->begin());
   ++NumPHIInsert;
   PhiToAllocaMap[PN] = AllocaNo;
```
Is this the direction you were hinting towards?

https://github.com/llvm/llvm-project/pull/83381