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

[Amara Emerson via llvm-commits]

aemerson wrote:

I like the idea overall. I'm just thinking of whether we should use a different name for this instead of overloading `combine`? I'm not strongly opposed  to it though. @arsenm @jayfoad ?

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