Unify the representation selection rules for NumberAdd/Subtract and
SpeculativeNumberAdd/Subtract wrt. Int32Add/Sub selection. We can
safely use Int32Add/Sub as long as the inputs are in the safe additive
integer range and the output is either truncated to Word32 or provably
in Signed32 or Unsigned32 range.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2253293005
Cr-Commit-Position: refs/heads/master@{#38746}