After fixing the memory barrier for maps (https://codereview.chromium.org/1714513003), we are using a temp register for the map case. The temp register should not be aliased with the stored value (otherwise we perform the mem barrier check with a wrong value). This CL makes sure it is not aliased.

BUG=chromium:590074
LOG=n

Review URL: https://codereview.chromium.org/1775083002

Cr-Commit-Position: refs/heads/master@{#34607}