[clang] [CIR] Upstream get_bitfield operation to load bit-field members from structs (PR #145971)

[Erich Keane via cfe-commits]

================
@@ -1669,6 +1669,85 @@ def GetGlobalOp : CIR_Op<"get_global",
   }];
 }
 
+//===----------------------------------------------------------------------===//
+// GetBitfieldOp
+//===----------------------------------------------------------------------===//
+
+def GetBitfieldOp : CIR_Op<"get_bitfield"> {
+  let summary = "Get a bitfield";
+  let description = [{
+    The `cir.get_bitfield` operation provides a load-like access to
+    a bit field of a record.
+
+    It expects a name if a bit field, a pointer to a storage in the
+    base record, a type of the storage, a name of the bitfield,
+    a size the bit field, an offset of the bit field and a sign.
+
+    A unit attribute `volatile` can be used to indicate a volatile load of the
+    bitfield.
+
+    Example:
+    Suppose we have a struct with multiple bitfields stored in
+    different storages. The `cir.get_bitfield` operation gets the value
----------------
erichkeane wrote:

```suggestion
    different members. The `cir.get_bitfield` operation gets the value
```

https://github.com/llvm/llvm-project/pull/145971