Don't check liveness of unallocatable registers.

Don't check liveness of unallocatable registers.

This includes registers like EFLAGS and ST0-ST7. We don't check for
liveness issues in the verifier and scavenger because registers will
never be allocated from these classes.

While in SSA form, we do care about the liveness of unallocatable
unreserved registers. Liveness of EFLAGS and ST0 neds to be correct for
MachineDCE and MachineSinking.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136541 91177308-0d34-0410-b5e6-96231b3b80d8