Implemented missing code for some LoadTypes.
Use unaligned instructions for I64 Loads.

Bug: v8:6600
Change-Id: I6ceb623005464c623eaa7512d3f1e4b0ebace86d
Reviewed-on: https://chromium-review.googlesource.com/939167Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#51590}