[llvm] [SimplifyLibCalls] fdim constant fold (PR #109235)

[Matt Arsenault via llvm-commits]

================
@@ -3109,6 +3109,52 @@ Value *LibCallSimplifier::optimizeRemquo(CallInst *CI, IRBuilderBase &B) {
   return ConstantFP::get(CI->getType(), Rem);
 }
 
+/// Constant folds fdim
+Value *LibCallSimplifier::optimizeFdim(CallInst *CI, IRBuilderBase &B) {
+  // Cannot perform the fold unless the call has attribute memory(none)
+  if (!CI->doesNotAccessMemory())
+    return nullptr;
+
+  // propogate poison/undef if any
+  if (match(CI->getArgOperand(0), m_Undef()))
+    return CI->getArgOperand(0);
+  if (match(CI->getArgOperand(1), m_Undef()))
+    return CI->getArgOperand(1);
----------------
arsenm wrote:

```suggestion
  if (isa<PoisonValue>(CI->getArgOperand(0))
    return CI->getArgOperand(0);
 if (isa<PoisonValue>(CI->getArgOperand(1))
    return CI->getArgOperand(1);
```

This isn't quite right. You can propagate poison -> poison, but undef needs more constraints (usually undef gets folded to qnan). For simplicity you can just handle poison

https://github.com/llvm/llvm-project/pull/109235