[AArch64][CodeGen] Fix of PR27158: incorrect peephole optimization in...

[AArch64][CodeGen] Fix of PR27158: incorrect peephole optimization in AArch64InstrInfo::optimizeCompareInstr

AArch64InstrInfo::optimizeCompareInstr has bug PR27158 which causes generation of incorrect code.
A compare instruction is substituted with another instruction which does not
produce the same flags as the original compare instruction.
This patch contains:
1. Fix of the bug.
2. A regression test in MIR.
3. A new test to check that SUBS is replaced by SUB.

Differential Revision: http://reviews.llvm.org/D18838

llvm-svn: 266969