Move function-live-in-handling code from the sdisel code to the scheduler.

Move function-live-in-handling code from the sdisel code to the scheduler.
This code should be emitted after legalize, so it can't be in sdisel.

Note that the EmitFunctionEntryCode hook should be updated to operate on the
DAG.  The X86 backend is the only one currently using this hook.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28315 91177308-0d34-0410-b5e6-96231b3b80d8