[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:

A vNi1 type is legal only on AVX512. But it's too trick to rely it in generic code. Adding virtual function is also not ideal. Maybe we can leave it as is. It's not a problem if register pressure is low. And we don't have a good way to know register pressure at this phase.

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