AMDGPU: Move trap lowering to DAG

Fixes traps in any block besides the entry block,
and fixes depending on a live-in physical register
by using a virtual register copy.

Also happens to stop emitting a nop in the case
debug trap is not supported.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301206 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed