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

[Phoebe Wang via llvm-commits]

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phoebewang wrote:

Or do it in `LowerADDSAT_SUBSAT` as @RKSimon suggested before.

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