[CIR][Lowering] fixes return value for = operator for bitfields (llvm#1247)

Basically that is - the return value for `=` operator for bitfield
assignment is wrong now. For example, the next function returns `7` for
3 bit bit field, though it should be `-1`:
```
int get_a(T *t) {
  return (t->a = 7);
}
```

This PR fix it. Actually, the bug was in the lowering - the integer cast
is applied in the wrong place (in comparison with the original codegen).

Author: gitoleg

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -3651,9 +3651,6 @@ mlir::LogicalResult CIRToLLVMSetBitfieldOpLowering::matchAndRewrite(
 
   auto resultTy = getTypeConverter()->convertType(op.getType());
 
-  resultVal = createIntCast(rewriter, resultVal,
-                            mlir::cast<mlir::IntegerType>(resultTy));
-
   if (info.getIsSigned()) {
     assert(size <= storageSize);
     unsigned highBits = storageSize - size;
@@ -3664,6 +3661,10 @@ mlir::LogicalResult CIRToLLVMSetBitfieldOpLowering::matchAndRewrite(
     }
   }
 
+  resultVal = createIntCast(rewriter, resultVal,
+                            mlir::cast<mlir::IntegerType>(resultTy),
+                            info.getIsSigned());
+
   rewriter.replaceOp(op, resultVal);
   return mlir::success();
 }

clang/test/CIR/CodeGen/bitfields.c

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
 // RUN: FileCheck --input-file=%t.cir %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t.ll
+// RUN: FileCheck --check-prefix=LLVM --input-file=%t.ll %s
 
 struct __long {
   struct __attribute__((__packed__)) {
@@ -131,6 +133,16 @@ void createD() {
   D d = {1,2,3};
 }
 
+// check the -1 is stored to the ret value
+// LLVM: define dso_local i32 {{@.*get_a.*}}
+// LLVM:    %[[V1:.*]] = alloca i32
+// LLVM:    store i32 -1, ptr %[[V1]], align 4
+// LLVM:    %[[V2:.*]] = load i32, ptr %[[V1]], align 4
+// LLVM:    ret i32 %[[V2:.*]]
+int get_a(T *t) {
+  return (t->a = 7);
+}
+
 typedef struct {
   int x : 15;
   int y ;