When suspending, rather than saving all registers up to a certain index,
only save the ones that are live according to the liveness analysis.
Others are saved as optimized out constants, and are skipped during the
GenaratorStore lowering. Symmetrically, only restore live registers when
resuming.

Change-Id: Icc2df905b0fe2fe5c372097bd67d5316edcd1b54
Reviewed-on: https://chromium-review.googlesource.com/905662
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51153}