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

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

Note: I only added the most basic and hopefully the most common patterns in this patch. Feel free to propose other patterns that you thought are important.

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