[MachineOutliner] NFC: Remove insertOutlinerPrologue, rename insertOutlinerEpilogue

insertOutlinerPrologue was not used by any target, and prologue-esque code was
beginning to appear in insertOutlinerEpilogue. Refactor that into one function,
buildOutlinedFrame.

This just removes insertOutlinerPrologue and renames insertOutlinerEpilogue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335076 91177308-0d34-0410-b5e6-96231b3b80d8