When initializing the stack state at a merge point, don't force all
stack slots into registers. Allow constants to stay constants as long
as they are not part of the merge. Otherwise we might break assumptions
of outer blocks which then try to merge a register into a constant and
fail.
Also, add some documentation to {InitMergeStackSlot} to document the
intent of the implementation.

R=titzer@chromium.org

Bug: v8:784050, v8:6600
Change-Id: I3a4c83b446909027be075d3207cb7c748a6b1aad
Reviewed-on: https://chromium-review.googlesource.com/766353Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49423}