If a Smi field load is truncated to Word32, we can just load the upper
32 bits on 64-bit architectures (with 32-bit Smis), which avoids the
nasty shift right that we'd have to perform otherwise to untag it.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2140553002
Cr-Commit-Position: refs/heads/master@{#37630}