[X86] Set AddPristinesAndCSRs to FixupBW LivePhysRegs. NFC.

We run after PEI, so we need to AddPristinesAndCSRs.
In practice, that makes no difference here, because we only ask about
liveness of super-registers of defined GR8/GR16 registers, so they
can't be pristine. Still, it's the correct thing to do.

Thanks to Quentin for noticing!

Follow-up to r267495.

llvm-svn: 267658