[PEI] Separate saving and restoring CSRs into different functions. NFC

Split insertCSRSpillsAndRestores into insertCSRSaves + insertCSRRestores.

This is mostly useful for future shrink-wrapping improvements where we
want to save / restore a specific part of the CSRs in a specific block.

Differential Revision: https://reviews.llvm.org/D35644

llvm-svn: 308554