Do most of the lowering in a pre-RA pass. Keep the skip jump
insertion late, plus a few other things that require more
work to move out.
One concern I have is now there may be COPY instructions
which do not have the necessary implicit exec uses
if they will be lowered to v_mov_b32.
This has a positive effect on SGPR usage in shader-db.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279464 91177308-0d34-0410-b5e6-96231b3b80d8
