[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]

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RKSimon wrote:

Would this work? It'd avoid the `ActiveBits > NarrowBits` early-out check
```suggestion
      for (unsigned NarrowBits = PowerOf2Ceil(ActiveBits); NarrowBits < ScalarBits; NarrowBits *= 2) {
```

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