Before this patch, Loads generated in the CSA on x64 that have a zero offset
displacement will add a zero to the effective address rather than using an
addressing mode that folds away the zero.

This functionality already exists on ia32, but the port wasn't purely mechanical
so it hadn't been done on x64.

R=epertoso@chromium.org
LOG=N

Review-Url: https://codereview.chromium.org/2602893002
Cr-Commit-Position: refs/heads/master@{#41974}