We were previously incorrectly changing:
  sub r0, 0, r1
  cmp r2, r0
  b.cond <addr>
to:
  cmn r2, r1
  b.cond <addr>

for all conditions. This is incorrect for conditions involving the C (carry)
and V (overflow) flags, and in particular in the case where r1 = INT_MIN.
The optimization is still safe to perform for Equal and NotEqual since they
do not depend on the C and V flags.

BUG=

Review-Url: https://codereview.chromium.org/2318043002
Cr-Commit-Position: refs/heads/master@{#39246}