By now the effect-control linearizer correctly determines the dominating
checkpoint, even for cases that contain effect-flow. We can elide the
temporary checkpoints during lowering of property loads and stores that
involve a sequence of map-checks.

R=bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2677483002
Cr-Commit-Position: refs/heads/master@{#42914}