On ia32, we were pinning too many registers, resulting in no unpinned
byte registers left (we only have three byte registers since {ebx}
is reserved for the root register).
It turns out that on most paths, we don't actually need to pin any
registers, since {Store} is often the last call for an operation (like
any store or set_global). If registers need to be pinned, only pass
those that must be kept alive across the {Store}. This allows to
compute a more narrow set of pinned registers on demand inside {Store}.

Plus minor drive-by changes.

R=titzer@chromium.org

Bug: chromium:894374, chromium:894307, v8:6600
Change-Id: Ic4d7131784c193dc7a2abf0e504d9973f6d5c5f1
Reviewed-on: https://chromium-review.googlesource.com/c/1275819
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56587}