This allows us to fuse the address computation with the actual memory
access operation on x64, which reduces the register pressure and the
number of instructions. There's probably some follow up cleanup that has
to happen to make sure the machine operator optimizations that are
relevant to word64 computations are also available (similar to what is
already available for word32).

R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2183043002
Cr-Commit-Position: refs/heads/master@{#38051}