Remove special-casing of return blocks for liveness.

Remove special-casing of return blocks for liveness.

Now that return value registers are return instruction uses, there is no
need for special treatment of return blocks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174416 91177308-0d34-0410-b5e6-96231b3b80d8