Oftentimes we can avoid the Smi check for ToBoolean truncations, since
we already know that the input is always going to be a HeapObject. So
introduce a dedicated TruncateTaggedPointerToBit operator, which uses
the fact that the input is known to be a HeapObject.

BUG=v8:5267
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2738483002
Cr-Commit-Position: refs/heads/master@{#43629}