[turbofan] Correctify JSToBoolean lowering.

[turbofan] Correctify JSToBoolean lowering.

Introduce a new AnyToBoolean simplified operator to handle the later
lowering of boolean conversions. Previously we tried to hack that with
the generic JSToBoolean, having its context set to zero, but that lead
to various problems/bugs and did not handle all cases.

TEST=cctest,unittests
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/800833003

Cr-Commit-Position: refs/heads/master@{#25958}