The register allocator has an optimization that makes it try reuse
spill-slots between phi-inputs and assign the same spill slot to a phi
output, if possible.
Furthermore, if the phi has no immediate register use, the output is
kept spilled.

With this change, no longer require that a phi input was spilled in an
immediate predecessor. Instead, it is good enough if it was spilled
at definition.

Change-Id: I317b9d5f9a9dda6a47972cdf84dc2627b996f3db
Reviewed-on: https://chromium-review.googlesource.com/c/1273053
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56594}