[InstCombine] Remove FastMathFlagGuard. NFC.

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -2674,10 +2674,9 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     // copysign Mag, (copysign ?, X) --> copysign Mag, X
     Value *X;
     if (match(Sign, m_Intrinsic<Intrinsic::copysign>(m_Value(), m_Value(X)))) {
-      IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
-      Builder.setFastMathFlags(II->getFastMathFlags() &
-                               cast<Instruction>(Sign)->getFastMathFlags());
-      Value *CopySign = Builder.CreateCopySign(Mag, X);
+      Value *CopySign = Builder.CreateCopySign(
+          Mag, X,
+          II->getFastMathFlags() & cast<Instruction>(Sign)->getFastMathFlags());
       return replaceInstUsesWith(*II, CopySign);
     }