[llvm] [X86] Narrow vXi32/vXi64 usubsat to vpsubusb/vpsubusw when LHS is known to fit in fewer bits (PR #206592)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

Basic principle is OK - currently wondering whether we should do this in legalization instead of a combine?

https://github.com/llvm/llvm-project/pull/206592