[clang] [CIR] Upstream StackSave and StackRestoreOp (PR #136426)

[Andy Kaylor via cfe-commits]

================
@@ -1419,6 +1419,50 @@ def CallOp : CIR_CallOpBase<"call", [NoRegionArguments]> {
     }]>];
 }
 
+//===----------------------------------------------------------------------===//
+// StackSave & StackRestoreOp
+//===----------------------------------------------------------------------===//
+
+def StackSaveOp : CIR_Op<"stacksave"> {
+  let summary = "remembers the current state of the function stack";
+  let description = [{
+    Remembers the current state of the function stack. Returns a pointer
+    that later can be passed into cir.stackrestore.
+    Useful for implementing language features like variable length arrays.
+
+    This operation corresponds to LLVM intrinsic `stacksave`.
+
+    ```mlir
+    %0 = cir.stacksave : <!u8i>
+    ```
+  }];
+
+  let results = (outs CIR_PointerType:$result);
+  let assemblyFormat = "attr-dict `:` qualified(type($result))";
+}
+
+def StackRestoreOp : CIR_Op<"stackrestore"> {
+  let summary = "restores the state of the function stack";
+  let description = [{
+    Restore the state of the function stack to the state it was
+    in when the corresponding cir.stacksave executed.
+    Useful for implementing language features like variable length arrays.
----------------
andykaylor wrote:

```suggestion
    This is used during the lowering of variable length array allocas.
```

https://github.com/llvm/llvm-project/pull/136426