When spill a range without register uses inside a loop, it is beneficial to spill the range ealier at the loop header to reduce memory moves from the back edges.

The changes to FindOptimalSpillingPos are motivated as follows:
 - Change “next_use->pos() < pos” to “next_use->pos() <= pos”.
  The former version causes a crash of mksnapshot in debug build,
  because it is possible that a UsePosition at a split point gets split
  to the previous range according to “DetachAt”. For example, we
  have a live range with:
    UseIntervals: [1, 20[
    UsePosition: 10
  When split the live range at position 10, we will get:
    Range 0:0: UseInterval: [1, 10[
               UsePosition: 10
    Range 0:1: UseInterval: [10, 20[

 - Change “NextUsePositionRegisterIsBenefitial” to
   “NextRegisterPosition”, because there’s always a
   “Define” use position at the loop header for those phis
   that do not require a register. Using the original check
   will hence not apply the optimization.


Change-Id: I3b0bb3687ba572f1d3fc1892cefae7e866d99baa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2094964Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Yolanda Chen <yolanda.chen@intel.com>
Cr-Commit-Position: refs/heads/master@{#66806}