The method {SpillAllRegisters} should really just spill registers, and
not also constants. Also, since more code is inlined into that method
now, we can optimize it to update register use counters only once in
the end, since no used register should be left after executing this
method.

R=titzer@chromium.org

Bug: v8:6600, chromium:802244
Change-Id: I737a1a87f8e912151062224952c4f5dffd43f802
Reviewed-on: https://chromium-review.googlesource.com/868022Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50644}