This moves the funky conditional code in ScheduleAndSelectInstructions to
the caller (the caller mostly knows statically what it wants).

My hidden agenda is to prepare the pipeline for post-scheduling passes.

Change-Id: Ia6009a6b2472d7211ef213eb4eb551957ae9709c
Bug: v8:5267
Reviewed-on: https://chromium-review.googlesource.com/863663
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50562}