Smi immediates are not supported, so instructions with Smi representations need their constants in a register. LAddI has already been doing this. The manifestation of the bug was that an operation would compute 0 instead of the correct result.
BUG=chromium:478612
LOG=y
R=verwaest@chromium.org
Review URL: https://codereview.chromium.org/1224623017
Cr-Commit-Position: refs/heads/master@{#29529}
