Allow the backward spilling heuristic to look and spill before the start
of the current range.

This solves a common scenario where control-flow aware allocation
cannot apply the heuristic as efficiently as splintering because it
creates smaller live ranges.

Bug: v8:9088
Change-Id: I7f8af8bb9c82849fbae0c652baa5011fd890690e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1776085
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63502}