[x86] improve SBB optimizations for SETB/SETA with subtract
This is another step towards removing a combine that turns sext
into select of constants and preparing the backend for an IR
future where select is the canonical form.
Earlier commits in this area:
https://reviews.llvm.org/rL306040
https://reviews.llvm.org/rL306072
https://reviews.llvm.org/rL307404 (https://reviews.llvm.org/D34652)
https://reviews.llvm.org/rL307471
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307821
91177308-0d34-0410-b5e6-
96231b3b80d8
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