[heap] Unify the immortal immovable root detection mechanism.

[heap] Unify the immortal immovable root detection mechanism.

Uniformly use the Heap::GetRootListIndex() and
Heap::RootIsImmortalImmovable() methods to detect immortal immovable
roots in the optimizing compilers.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1178853002

Cr-Commit-Position: refs/heads/master@{#28933}