This change adds two spilling modes: SpillAtDefinition and SpillDeferred.
The former is the known spilling mode where we spill at definition. The
latter spills only in deferred code regions. This is implemented based on
control flow aware allocation and its invariants.

The effect is mostly the same as splintering with the exception of
forward looking allocation decisions still being impacted by register
constraints in deferred code.

Change-Id: Ia708e5765dd095196a8127deb2d8bec950d37e04
Reviewed-on: https://chromium-review.googlesource.com/c/1437118Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59759}