[ARM] Fix perf regression in compare optimization.

Fix a performance regression caused by r322737.

While trying to make it easier to replace compares with existing adds and
subtracts, I accidentally stopped it from doing so in some cases.  This should
fix that.  I'm also fixing another potential bug in that commit.

Differential Revision: https://reviews.llvm.org/D42263

llvm-svn: 322972