JIT: optimize Enum.HasFlag (#13748)

Check for calls to `Enum.HasFlag` using the new named intrinsic support
introduced in #13815. Implement a simple recognizer for these named
intrinsics (currently just recognizing `Enum.HasFlag`).

When the call is recognized, optimize if both operands are boxes with
compatible types and both boxes can be removed. The optimization changes the
call to a simple and/compare tree on the underlying enum values.

To accomplish this, generalize the behavior of `gtTryRemoveBoxUpstreamEffects`
to add a "trial removal" mode and to optionally suppress narrowing of the
copy source.

Invoke the optimization during importation (which will catch most cases) and
again during morph (to get the post-inline cases).

Added test cases. Suprisingly there were almost no uses of HasFlag in the
current CoreCLR test suite.

Closes #5626.

1 files changed, 16 insertions, 0 deletions

diff --git a/src/jit/namedintrinsiclist.h b/src/jit/namedintrinsiclist.h
new file mode 100644
index 0000000000..cf81afc119
--- /dev/null
+++ b/src/jit/namedintrinsiclist.h
@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#ifndef _NAMEDINTRINSICLIST_H_
+#define _NAMEDINTRINSICLIST_H_
+
+// Named jit intrinsics
+
+enum NamedIntrinsic
+{
+    NI_Illegal      = 0,
+    NI_Enum_HasFlag = 1
+};
+
+#endif // _NAMEDINTRINSICLIST_H_