[CIR][CIRGen] Implement Nullpointer arithmatic extension (llvm#861)

See the test for example.

Author: ChuanqiXu9

clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp

@@ -1245,7 +1245,7 @@ static mlir::Value buildPointerArithmetic(CIRGenFunction &CGF,
   //
   if (BinaryOperator::isNullPointerArithmeticExtension(
           CGF.getContext(), op.Opcode, expr->getLHS(), expr->getRHS()))
-    llvm_unreachable("null pointer arithmetic extension is NYI");
+    return CGF.getBuilder().createIntToPtr(index, pointer.getType());
 
   // Differently from LLVM codegen, ABI bits for index sizes is handled during
   // LLVM lowering.

clang/test/CIR/CodeGen/null-arithmatic-expression.c

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-cir %s -o - | FileCheck %s
+
+#define NULL ((void *)0)
+
+char *foo() {
+  return (char*)NULL + 1;
+}
+
+// CHECK:  cir.func no_proto @foo()
+// CHECK:    [[CONST_1:%[0-9]+]] = cir.const #cir.int<1> : !s32i
+// CHECK:    {{.*}} = cir.cast(int_to_ptr, [[CONST_1]] : !s32i)
+// CHECK:    cir.return