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=v8:4614
TBR=bmeurer@chromium.org
LOG=n

Review-Url: https://codereview.chromium.org/1947673002
Cr-Commit-Position: refs/heads/master@{#35998}