RegisterScavenging: Move scavenging logic from PEI to RegisterScavenging; NFC

These parts do not depend on any PrologEpilogInserter logic and
therefore better fits RegisterScaveging.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304596 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed