[llvm] [AMDGPU] Implement IR variant of isFMAFasterThanFMulAndFAdd (PR #121465)

[Chinmay Deshpande via llvm-commits]

================
@@ -16992,6 +17021,33 @@ bool SITargetLowering::checkForPhysRegDependency(
   return false;
 }
 
+/// Check if it is profitable to hoist instruction in then/else to if.
+bool SITargetLowering::isProfitableToHoist(Instruction *I) const {
+  if (!I->hasOneUse())
+    return true;
+
+  Instruction *User = I->user_back();
+  // TODO: Add more patterns that are not profitable to hoist
+  switch (I->getOpcode()) {
+  case Instruction::FMul: {
+    if (User->getOpcode() != Instruction::FSub &&
+        User->getOpcode() != Instruction::FAdd)
+      return true;
+
+    const TargetOptions &Options = getTargetMachine().Options;
+    const Function *F = I->getFunction();
+
+    return ((!I->hasAllowContract() || !User->hasAllowContract()) &&
+            Options.AllowFPOpFusion != FPOpFusion::Fast &&
+            !Options.UnsafeFPMath) ||
----------------
chinmaydd wrote:

I believe existing code takes care of this. This is without pushing the demorgan through -

```c++
return !((I->hasAllowContract() && User->hasAllowContract()) ||
         Options.AllowFPOpFusion == FPOpFusion::Fast ||
         Options.UnsafeFPMath) &&
         isFMAFasterThanFMulAndFAdd(*F, User->getType());
```

https://github.com/llvm/llvm-project/pull/121465