We can compute the absolute integer value w/o any conditional execution
by using the bit trick formula

  let sign = input >> 31 in
  (input ^ sign) - sign

which generates fairly decent code on all supported architectures.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2169293002
Cr-Commit-Position: refs/heads/master@{#37965}