Adds TestResultScope and uses it to directly jump/fall through to the
correct branch in expressions used as branch conditions.
Should enable nicer TurboFan-graphs for easier control-flow
transformations in the future.

BUG=v8:4280
LOG=n

Review-Url: https://codereview.chromium.org/2242463002
Cr-Commit-Position: refs/heads/master@{#38634}