AMDGPU: Fix not expanding control flow after some kill blocks

Also stop trying to insert skip blocks at end_cf. This
was inserting them at the end of the block which doesn't make
sense. The skip should be inserted at the beginning of the block
right after the end cf. Just remove this for now since no tests
seem to stress this and I think this can be handled more generally
later.

Fixes bug 28550

llvm-svn: 275510