[CIR] Implement GenericSelectionExpr (#1718)

Support codgen for GenericSelectionExpr

Author: AmrDeveloper

clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp

@@ -158,7 +158,7 @@ class ScalarExprEmitter : public StmtVisitor<ScalarExprEmitter, mlir::Value> {
     return Visit(E->getReplacement());
   }
   mlir::Value VisitGenericSelectionExpr(GenericSelectionExpr *GE) {
-    llvm_unreachable("NYI");
+    return Visit(GE->getResultExpr());
   }
   mlir::Value VisitCoawaitExpr(CoawaitExpr *S) {
     return CGF.emitCoawaitExpr(*S).getScalarVal();

clang/test/CIR/CodeGen/generic-selection.c

@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s -check-prefix=CIR
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t.ll
+// RUN: FileCheck --input-file=%t.ll %s -check-prefix=LLVM
+
+void foo() {
+  int a;
+  int r = _Generic(a, double: 1, float: 2, int: 3, default: 4);
+}
+  
+// CIR: %[[A:.*]] = cir.alloca !s32i, !cir.ptr<!s32i>, ["a"]
+// CIR: %[[RES:.*]] = cir.alloca !s32i, !cir.ptr<!s32i>, ["r", init]
+// CIR: %[[RES_VAL:.*]] = cir.const #cir.int<3> : !s32i
+// CIR: cir.store{{.*}} %[[RES_VAL]], %[[RES]] : !s32i, !cir.ptr<!s32i>
+
+// LLVM: %[[A:.*]] = alloca i32, i64 1, align 4
+// LLVM: %[[RES:.*]] = alloca i32, i64 1, align 4
+// LLVM: store i32 3, ptr %[[RES]], align 4