[llvm] [X86] Add pass to coalesce redundant vector constant materializations (PR #211619)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

#212950 should have helped a lot with the SET0 patterns - it just needs some better handling on SUBREG_TO_REG rematerialization.

https://github.com/llvm/llvm-project/pull/211619