[CIR][CodeGen] Add initial support for comma operator

Author: cmarcelo

clang/lib/CIR/CIRGenExprScalar.cpp

@@ -328,7 +328,9 @@ class ScalarExprEmitter : public StmtVisitor<ScalarExprEmitter, mlir::Value> {
   mlir::Value VisitBinLAnd(const BinaryOperator *E) { llvm_unreachable("NYI"); }
   mlir::Value VisitBinLOr(const BinaryOperator *E) { llvm_unreachable("NYI"); }
   mlir::Value VisitBinComma(const BinaryOperator *E) {
-    llvm_unreachable("NYI");
+    CGF.buildIgnoredExpr(E->getLHS());
+    // NOTE: We don't need to EnsureInsertPoint() like LLVM codegen.
+    return Visit(E->getRHS());
   }
 
   mlir::Value VisitBinPtrMemD(const Expr *E) { llvm_unreachable("NYI"); }

clang/test/CIR/CodeGen/comma.cpp

@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fenable-clangir -Wno-unused-value -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s
+
+int c0() {
+    int a = 1;
+    int b = 2;
+    return b + 1, a;
+}
+
+// CHECK: cir.func @_Z2c0v() -> i32 {
+// CHECK: %[[#RET:]] = cir.alloca i32, cir.ptr <i32>, ["__retval", uninitialized]
+// CHECK: %[[#A:]] = cir.alloca i32, cir.ptr <i32>, ["a", cinit]
+// CHECK: %[[#B:]] = cir.alloca i32, cir.ptr <i32>, ["b", cinit]
+// CHECK: %[[#LOADED_B:]] = cir.load %[[#B]] : cir.ptr <i32>, i32
+// CHECK: %[[#]] = cir.binop(add, %[[#LOADED_B]], %[[#]]) : i32
+// CHECK: %[[#LOADED_A:]] = cir.load %[[#A]] : cir.ptr <i32>, i32
+// CHECK: cir.store %[[#LOADED_A]], %[[#RET]] : i32, cir.ptr <i32>