[AMDGPU] Fix read-undef flags when schedule is reverted

If two subregs of the same register are defined and we need to revert
schedule changing def order, we will end up with both instructions
having def,read-undef flags because adjustLaneLiveness() will only set
this flag but will not remove it.

Fix this by removing read-undef flags before calling adjustLaneLiveness.

Differential Revision: https://reviews.llvm.org/D30428

llvm-svn: 296484