Fixes issues raised after committing the following CLs:

    https://codereview.chromium.org/1649053002
    https://codereview.chromium.org/1647113003

Also generates a trap instruction if hybrid assembly is turned off, and
the skip implementation command line flag is turned on.

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1654483002 .