This fixes the checks of accumulator usage flags in the computation of
the interpreter register liveness during bytecode analysis. The usage
flags at hand are bit patterns as opposed to flat enum values. Use the
safe accessors instead of plain comparison.

R=jarin@chromium.org
TEST=mjsunit/regress/regress-crbug-683581
BUG=chromium:683581

Review-Url: https://codereview.chromium.org/2651653005
Cr-Commit-Position: refs/heads/master@{#42648}