[AArch64] Don't optimize all compare instructions.

"optimizeCompareInstr" converts compares (cmp/cmn) into plain sub/add
instructions when the flags are not used anymore. This conversion is valid for
most instructions, but not all. Some instructions that don't set the flags
(e.g. sub with immediate) can set the SP, whereas the flag setting version uses
the same encoding for the "zero" register.

Update the code to also check for the return register before performing the
optimization to make sure that a cmp doesn't suddenly turn into a sub that sets
the stack pointer.

I don't have a test case for this, because it isn't easy to trigger.

llvm-svn: 222255