[X86] Remove isSafeToClobberEFLAGS helper and just inline it into the call sites.

This is just a thin wrapper around computeRegisterLivness which
we can just call directly. The only real difference is that
isSafeToClobberEFLAGS returns a bool and computeRegisterLivness
returns an enum. So we need to check for the specific enum value
that isSafeToClobberEFLAGS was hiding.

I've also adjusted which sites pass an explicit value for
Neighborhood since the default for computeRegisterLivness is 10.