[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]

================

----------------
RKSimon wrote:

Ensure the vXiN correctly aliases with the smaller type
```suggestion
      if (NarrowBits && (ScalarBits % NarrowBits) == 0) {
```

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