AMDGPU: Fix crash when scheduling DBG_VALUE

This calls handleMove with a DBG_VALUE instruction,
which isn't tracked by LiveIntervals. I'm not sure
this is the correct place to fix this. The generic
scheduler seems to have more deliberate region
selection that skips dbg_value.

The test is also really hard to reduce. I haven't been able
to figure out what exactly causes this particular case to
try moving the dbg_value.

llvm-svn: 319732