Before this change we would first load an 8/16/32-bit value from memory into a 32-bit register, then zero/sign-extend from that register to a 64-bit one. Now we replace that pattern with a single movsx/movzx.

Ported from http://crrev.com/2183923003

R=bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2220483003
Cr-Commit-Position: refs/heads/master@{#38388}