I previously wrote a comment that said "We haven't seen any indication
of performance improvements from seeking optimal spilling positions
except on loop-top phi values". That statement is no longer true, now
that I've looked a little harder. In the latest version of the Mono
interpreter, we can improve performance by 2.5% by enabling SpillPlacer
for any value defined within a loop.

Bug: v8:10606
Change-Id: I25e06458c87ad4ffcefe52be3042032e05a47b35
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2381557Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#69646}