ARM: fix handling of SUB immediates in peephole opt.

We were negating an immediate that was going to be used in a SUBri form
unnecessarily. Since ADD/SUB are very similar we *can* do that, but we have to
change the SUB to an ADD at the same time. This also applies to ADD, and allows
us to handle a slightly larger range of immediates for those two operations.

rdar://25992245

llvm-svn: 268276