[X86] Add test cases that exercise the BSR/BSF optimization combineCMov.

combineCmov tries to remove compares against BSR/BSF if we can prove the input to the BSR/BSF are never zero.

As far as I can tell most of the time codegenprepare despeculates ctlz/cttz and gives us a cttz_zero_undef/ctlz_zero_undef which don't use a cmov.

So the only way I found to trigger this code is to show codegenprepare an illegal type which it won't despeculate.

I think we should be turning ctlz/cttz into ctlz_zero_undef/cttz_zero_undef for these cases before we ever get to operation legalization where the cmov is created. But wanted to add these tests so we don't regress.

llvm-svn: 324409