If a HeapObject field is 8 bytes long and 8 bytes aligned, but we use only four bytes of it
on little endian architectures we will access to lower 4 bytes of the field using the same
base address as accessing the whole eight bytes
On big-endian architectures however we use base address to access the full 8 bytes, but base
address + 4 to access to lower 4 bytes. For this reason, the assert in OffsetForFieldAccess
fails on MIPS64 big endian.
We fix the issue by removing the assert that checks that offset is pointer size aligned.

TEST=mjsunit/regress/regress-crbug-648737
BUG=

Review-Url: https://codereview.chromium.org/2453333002
Cr-Commit-Position: refs/heads/master@{#40772}