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

[Matt Arsenault 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
----------------
arsenm wrote:

Mention source modifiers 

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