[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

llvm-svn: 307821