Emit code like this

  cmpq reg, [kRootRegister + offset]

when selecting instructions for comparisons with immortal, immovable
roots. This reduces register pressure as we don't need to load those
roots into registers first.

R=jarin@chromium.org
TBR=hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2396923003
Cr-Commit-Position: refs/heads/master@{#40022}