At this point, last_index is definitely a canonicalized non-negative
number, which implies that any non-Smi last_index is greater than
the maximal string length. That in turn means that the matcher will
definitely fail, and we can avoid the expensive runtime call.

BUG=v8:6365,v8:6344

Review-Url: https://codereview.chromium.org/2863643004
Cr-Commit-Position: refs/heads/master@{#45143}