MachineFrameInfo: Cleanup some parameter naming inconsistencies; NFC

Consistently use the same parameter names as the names of the affected
fields. This avoids some unintuitive abbreviations like `isSS`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319722 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed