This refactors logic for handling IfStatement and Conditional nodes (including
block-coverage related slot and counter creation) into a new control-flow
builder.

Bug: v8:6000
Change-Id: Ib5b1724bdf8571fb55d310be79cc60dcf5473b81
Reviewed-on: https://chromium-review.googlesource.com/579509Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46818}