Instead of always passing the MachineType, we can often just pass the
accessed memory size or the MachineRepresentation, which is less
information to pass and will simplify the upcoming refactoring for
memory operations in Liftoff.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I8748f8e00dcfdbc4082893143fe88bdafde99053
Reviewed-on: https://chromium-review.googlesource.com/822194Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50041}