[llvm] [SimplifyLibCalls] Move constant folding logic in `ConstantFoldLibCall2` (PR #172139)

[Matt Arsenault via llvm-commits]

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@@ -3241,6 +3241,15 @@ static Constant *ConstantFoldLibCall2(StringRef Name, Type *Ty,
     if (TLI->has(Func))
       return ConstantFoldBinaryFP(atan2, Op1V, Op2V, Ty);
     break;
+  case LibFunc_fdim:
+  case LibFunc_fdimf:
+  case LibFunc_fdiml:
+    APFloat Difference = Op1V;
+    Difference.subtract(Op2V, RoundingMode::NearestTiesToEven);
+
+    APFloat MaxVal =
+      maximum(Difference, APFloat::getZero(Ty->getFltSemantics()));
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arsenm wrote:

The actual implementation logic should probably be in APFloat 

https://github.com/llvm/llvm-project/pull/172139