[x86] add special-case lowering for usubsat for pre-SSE4

usubsat X, SMIN --> (X ^ SMIN) & (X s>> BW-1)

This would be a regression with D112085 where we combine to
usubsat more aggressively, so avoid that by matching the
special-case where we are subtracting SMIN (signmask):
https://alive2.llvm.org/ce/z/4_3gBD

Differential Revision: https://reviews.llvm.org/D112095