[ConstantFold] Special case log1p +/-0.0

C's Annex F specifies that log1p +/-0.0 returns the input value;
however, this behavior is optional and host C libraries may behave
differently. This change applies the Annex F behavior to constant
folding by LLVM.

Author: hubert-reinterpretcast

llvm/lib/Analysis/ConstantFolding.cpp

@@ -2407,6 +2407,9 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
       break;
     case LibFunc_log1p:
     case LibFunc_log1pf:
+      // Implement optional behavior from C's Annex F for +/-0.0.
+      if (U.isZero())
+        return ConstantFP::get(Ty->getContext(), U);
       if (APF > APFloat::getOne(APF.getSemantics(), true) && TLI->has(Func))
         return ConstantFoldFP(log1p, APF, Ty);
       break;