[turbofan] Remove obsolete contains_js_nodes_ field.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/806653003

Cr-Commit-Position: refs/heads/master@{#25811}
[turbofan] Correctify TruncateFloat64ToInt32 reduction in MachineOperatorReducer.

TEST=unittests
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/801263002

Cr-Commit-Position: refs/heads/master@{#25812}