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

[via llvm-commits]

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sipher-01 wrote:

 Done 
 - Updated the condition to `ScalarBits > 8 && isPowerOf2_32(ScalarBits) && !TLI.isOperationLegal(ISD::USUBSAT, VT)`, removed the early return, dropped the redundant divisibility check, and clang-formatted.
 - Path B removed for now ( should I start that as separate issue? ).
 - Also updated the pr description as per the solution implemented. 

 All X86 codegen tests passing.

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