RAFast: Generalize the logic for return operands.

This removes implicit assumption about the form of MI coming into regalloc. In particular, it should be independent of ProcessImplicitDefs which will eventually become a standard part of coming out of SSA--unless we simply can eliminate IMPLICIT_DEF completely. Current unit tests expose this once I remove incidental pass ordering restrictions.

This is not a final fix. Just a temporary workaround until I figure out the right way.

llvm-svn: 149360