[llvm] [AMDGPU][TargetLowering] Allow forming overflow op if it is legal (PR #156266)

[Matt Arsenault via llvm-commits]

================
@@ -3455,6 +3455,10 @@ class LLVM_ABI TargetLoweringBase {
   /// matching of other patterns.
   virtual bool shouldFormOverflowOp(unsigned Opcode, EVT VT,
                                     bool MathUsed) const {
+    // Form it if it is legal.
+    if (isOperationLegal(Opcode, VT))
+      return true;
+
----------------
arsenm wrote:

I don't understand the existing logic here, the's a random special case with a todo. Can you just delete the opcode check? I also have no idea what "MathUsed" is supposed to mean 

https://github.com/llvm/llvm-project/pull/156266