This is a reland of ada64800, fixed
for 32 bit architectures (register pairs).

Original change's description:
> [Liftoff] Fix register use count
>
> 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/1111958
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53976}

TBR=titzer@chromium.org

Bug: chromium:854050, v8:6600
Change-Id: Ibc8801737e9604a8490382c569b0378585625376
Reviewed-on: https://chromium-review.googlesource.com/1112238
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53981}