Tracking labels for most of these statements made no difference: only
try-statements require the special treatment of being wrapped in a
block. The previous code existed to support strong mode, which is
long gone.

This also results in a tiny regression of the error message for
a labelled `continue` statement targeting itself, but I'm not
convinced that anyone would ever intend to label a continue
statement (and Chakra and SpiderMonkey give similarly inaccurate
error messages for this case).

This is effectively a revert of d8bccfe9.

Bug: v8:6092
Change-Id: I25b62e10f6a20597e9686f08df76ba9724249618
Reviewed-on: https://chromium-review.googlesource.com/653380Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47904}