AMDGPU: Fix debug info handling in post-RA bundler

This was allowing debug instructions to break the bundling, which
would change scheduling behavior. Bundle debug info / kills inside
the bundle. This seems to work OK, although the asm printer doesn't
understand these in a bundle. This implicitly expects the memory
legalizer to unbundle. It would probably be slightly nicer to move
these after.

Rewrite the loop to be clearer and make sure we don't end a bundle on
a meta instruction, only allow them in between other valid bundle
instructions.