[InstCombine] Return FRem, as opposed to substituteInParent.

This attempts to fix the ASan buildbot, which is detecting that CI is used
after it is removed in substituteInParent. The idea was to make sure it was
removed even if it had side-effects writing errno, but that appears to happen
if we return FRem directly as usual.

Author: davemgreen

llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

@@ -2820,7 +2820,7 @@ Value *LibCallSimplifier::optimizeFMod(CallInst *CI, IRBuilderBase &B) {
     Value *FRem = B.CreateFRemFMF(CI->getOperand(0), CI->getOperand(1), CI);
     if (auto *FRemI = dyn_cast<Instruction>(FRem))
       FRemI->setHasNoNaNs(true);
-    substituteInParent(CI, FRem);
+    return FRem;
   }
   return nullptr;
 }