Fix -Wunused-variable in SemaAMDGPU.cpp in release build, NFC

Author: hokein

clang/lib/Sema/SemaAMDGPU.cpp

@@ -31,7 +31,8 @@ bool SemaAMDGPU::CheckAMDGCNBuiltinFunctionCall(unsigned BuiltinID,
     constexpr const int SizeIdx = 2;
     llvm::APSInt Size;
     Expr *ArgExpr = TheCall->getArg(SizeIdx);
-    ExprResult R = SemaRef.VerifyIntegerConstantExpression(ArgExpr, &Size);
+    [[maybe_unused]] ExprResult R =
+        SemaRef.VerifyIntegerConstantExpression(ArgExpr, &Size);
     assert(!R.isInvalid());
     switch (Size.getSExtValue()) {
     case 1: