In {SetLocalFromStackSlot}, we decrement the use count of the register
in the target slot without updating this slot, and then call
{GetUnusedRegister}. At that point, the register use counts do not
match the cache state, which leads to errors later on.
This CL fixes this by marking the target slot as a stack slot after
reducing the register use count.

It also adds a Validation which helped to find that error and will
catch similar errors earlier.

R=titzer@chromium.org

Bug: chromium:854050, v8:6600
Change-Id: I74d3a5aa947ec4247d7b4557567f642bf4082316
Reviewed-on: https://chromium-review.googlesource.com/1111958Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53976}