According to new store IC calling convention the value, slot and vector are passed
on the stack and there's no need in trying to preserve values or respective registers
in store handlers.

Nice bonus: we also don't need virtual registers anymore.

BUG=v8:5407

Review-Url: https://codereview.chromium.org/2357323003
Cr-Commit-Position: refs/heads/master@{#39672}