[llvm] [NFC][TableGen][GlobalISel] Move MIR pattern parsing out of combiner (PR #86789)

[Matt Arsenault via llvm-commits]

================
@@ -0,0 +1,462 @@
+//===- PatternParser.cpp ----------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Common/GlobalISel/PatternParser.h"
+#include "Basic/CodeGenIntrinsics.h"
+#include "Common/CodeGenTarget.h"
+#include "Common/GlobalISel/CombinerUtils.h"
+#include "Common/GlobalISel/Patterns.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/PrettyStackTrace.h"
+#include "llvm/Support/SaveAndRestore.h"
+#include "llvm/TableGen/Error.h"
+#include "llvm/TableGen/Record.h"
+
+namespace llvm {
+namespace gi {
+static constexpr StringLiteral MIFlagsEnumClassName = "MIFlagEnum";
+
+namespace {
+class PrettyStackTraceParse : public PrettyStackTraceEntry {
+  const Record &Def;
+
+public:
+  PrettyStackTraceParse(const Record &Def) : Def(Def) {}
+
+  void print(raw_ostream &OS) const override {
+    if (Def.isSubClassOf("GICombineRule"))
+      OS << "Parsing GICombineRule '" << Def.getName() << "'";
+    else if (Def.isSubClassOf(PatFrag::ClassName))
+      OS << "Parsing " << PatFrag::ClassName << " '" << Def.getName() << "'";
+    else
+      OS << "Parsing '" << Def.getName() << "'";
+    OS << '\n';
----------------
arsenm wrote:

```suggestion
    if (Def.isSubClassOf("GICombineRule"))
      OS << "Parsing GICombineRule '" << Def.getName() << '\'';
    else if (Def.isSubClassOf(PatFrag::ClassName))
      OS << "Parsing " << PatFrag::ClassName << " '" << Def.getName() << '\'';
    else
      OS << "Parsing '" << Def.getName() << '\'';
    OS << '\n';
```

https://github.com/llvm/llvm-project/pull/86789