[llvm] [GlobalISel] Add `combine` action for C++ combine rules (PR #135941)

[Sergei Barannikov via llvm-commits]

================
@@ -370,6 +370,42 @@ The following expansions are available for MIR patterns:
     (match (G_ZEXT $root, $src):$mi),
     (apply "foobar(${root}.getReg(), ${src}.getReg(), ${mi}->hasImplicitDef())")>;
 
+``combine`` Operator
+~~~~~~~~~~~~~~~~~~~~
+
+``GICombineRule`` also supports a single ``combine`` pattern, which is a shorter way to
+declare patterns that just match one or more instruction, then defer all remaining matching
+and rewriting logic to C++ code.
+
+.. code-block:: text
+  :caption: Example usage of the combine operator.
+
+  // match + apply
+  def FooLong : GICombineRule<
+    (defs root:$root),
+    (match (G_ZEXT $root, $src):$mi, "return matchFoo(${mi});"),
+    (apply "applyFoo(${mi});")>;
+
+  // combine
+  def FooShort : GICombineRule<
+    (defs root:$root),
+    (combine (G_ZEXT $root, $src):$mi, "return combineFoo(${mi});")>;
----------------
s-barannikov wrote:

Just to clarify, is it different from
```
def FooShort : GICombineRule<
  (defs root:$root),
  (match (G_ZEXT $root, $src):$mi),
  (apply "return combineFoo(${mi});")
>;
```
?

https://github.com/llvm/llvm-project/pull/135941