simplify result value lowering by splitting the selection of *where* to return

simplify result value lowering by splitting the selection of *where* to return
registers out from the logic of *how* to return them.

This changes X86-64 to mark EAX live out when returning a 32-bit value,
where before it marked RAX liveout.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34582 91177308-0d34-0410-b5e6-96231b3b80d8