The vast majority of blocks we create in the parser have no associated
labels, so it seems silly to waste a pointer on labels_ for all such
blocks.

This is accomplished by delegating responsibility for labels storage to
each subclass of BreakableStatement, and then further-specializing Block
by creating a new subclass, LabeledBlock.

Bug: v8:6092
Change-Id: I88c824639254e5890b25a86cc156bfc4310bf2b1
Reviewed-on: https://chromium-review.googlesource.com/639063Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47689}