This change lands a change to the register allocator. Other than
classical linear scan, we now take control flow events into account
when deciding spill descisions.

The basic idea is to restore the state of a predecessor on entry
of a successor. In the case of multiple predecessors, we use
heuristics to compute the new state based on the predecessors.

The main addition to the machinery is to support unspilling
live ranges and to undo live range splitting in certain cases.

Currently, disabled behind a flag.

Change-Id: I37a70c948be826a90d8b806a52856ad81f475573
Reviewed-on: https://chromium-review.googlesource.com/c/1426129
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59195}