In order to implement CacheState merging correctly, we need to know at
least the register type to be used for each stack slot. For stack
slots, this is not stored currently. Since there is enough space
available in the VarState anyway, we just always store the full type,
which will allow also for other optimizations like not always spilling
and filling the full 8 bytes.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I6c7c1d39847063591bf72b7f186a2128295d889b
Reviewed-on: https://chromium-review.googlesource.com/789861
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49641}