[x86] avoid false dependency stall on 'sbb' with same source reg

[x86] avoid false dependency stall on 'sbb' with same source reg

This is effectively inverting the transform added with D116804
because the downside of the false dependency of something like
"sbb %eax, %eax" is much greater than the upside of eliminating
a zeroing instruction on (all?) Intel CPUs.

Differential Revision: https://reviews.llvm.org/D118843