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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@308554 91177308-0d34-0410-b5e6-96231b3b80d8

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