[llvm] [SelectionDAG] Introducing the SelectionDAG pattern matching framework (PR #78654)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> Random question - will this play nice with the template approach taken for VP node (e.g. #80105)?

My apologies for the delay.
I think supporting `VPMatchContext::match` in SDPatternMatch makes a lot of sense and I'll add that. Though I'm not sure if we should bring `VPMatchContext::getNode` into SDPatternMatch as well.

https://github.com/llvm/llvm-project/pull/78654