port af677d7b (r35998)

  original commit message:
  https://codereview.chromium.org/1938213002 added xchg_b and xchg_w, but didn't
  add it to the disassembler, and there were no tests that caught it.

BUG=

Review-Url: https://codereview.chromium.org/1947843002
Cr-Commit-Position: refs/heads/master@{#36051}