[llvm] [SLP]Initial compatibility support for shl v, 1 and add v, v (PR #181168)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

flyby question (sorry I don't have much time at the moment) but do we need to be careful about shl(undef,1) -> add(undef,undef) cases?

https://github.com/llvm/llvm-project/pull/181168