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

[Amara Emerson 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
----------------
aemerson wrote:

s/instruction/instructions

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