[AArch64][CodeGen] NFC refactor AArch64InstrInfo::optimizeCompareInstr to...

[AArch64][CodeGen] NFC refactor AArch64InstrInfo::optimizeCompareInstr to prepare it for fixing a bug in it

AArch64InstrInfo::optimizeCompareInstr has a bug which causes generation of incorrect code (PR#27158).
The patch refactors the function to simplify reviewing the fix of the bug.

1. Function name ‘modifiesConditionCode’ is changed to ‘areCFlagsAccessedBetweenInstrs’
   to reflect that the function can check modifying accesses, reading accesses or both.
2. Function ‘AArch64InstrInfo::optimizeCompareInstr’
   - Documented the function
   - Cmp_NZCV is DeadNZCVIdx to reflect that it is an operand index of dead NZCV
   - The code for the case of substituting CmpInstr is put into separate
     functions the main of them is ‘substituteCmpInstr’.

Differential Revision: http://reviews.llvm.org/D18609

llvm-svn: 265531