[turbofan] Improve JSToBoolean typing.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/792013006

Cr-Commit-Position: refs/heads/master@{#25896}
Remove obsolete V8_INFINITY macro.

Use std::numeric_limits consistently.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/798413003

Cr-Commit-Position: refs/heads/master@{#25897}