For determining whether unaligned memory accessed is allowed, it is
actually sufficient to pass the MachineRepresentation instead of the
MachineType.
As the MachineRepresentation is an enum, this allows to store the set
of unsupported types as an EnumSet instead of std::vector.

R=mstarzinger@chromium.org

Change-Id: Ibe44717bae3a6034bae683057e80bd40d92e70ae
Reviewed-on: https://chromium-review.googlesource.com/598790
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47123}