AMDGPU: Remove FixControlFlowLiveIntervals pass

This hasn't done anything in a long time. This was
running after the the control flow pseudos were expanded,
so this would never find them. The control flow pseudo
expansion was moved to solve the problem this pass was
supposed to solve in the first place, except handling
it earlier also fixes it for fast regalloc which doesn't
use LiveIntervals.

Noticed by checking LCOV reports.

llvm-svn: 310274