[X86] Return Op instead of SDValue() for lowering flags_read/write in…

…trinsics

Returning SDValue() means we didn't handle it and the common
code should try to expand it. But its a target intrinsic so
expanding won't do anything and just leave the node alone. But
it will print confusing debug messages.

By returning Op we tell the common code that the node is legal
and shouldn't receive any further processing.

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -24105,7 +24105,7 @@ static SDValue LowerINTRINSIC_W_CHAIN(SDValue Op, const X86Subtarget &Subtarget,
       MFI.setHasCopyImplyingStackAdjustment(true);
       // Don't do anything here, we will expand these intrinsics out later
       // during FinalizeISel in EmitInstrWithCustomInserter.
-      return SDValue();
+      return Op;
     }
     case Intrinsic::x86_lwpins32:
     case Intrinsic::x86_lwpins64: