Equality with null/undefined is equivalent to a check on the undetectable bit
on the map of the object. This would be more efficient than performing the entire
comparison operation.

This cl introduces:
1. A new bytecode called TestUndetectable that checks if the object is null/undefined.
2. Updates peeophole optimizer to emit TestUndetectable when a LdaNull/Undefined
precedes equality check.
4. TestUndetectable is transformed to ObjectIsUndetectable operator when building
turbofan graph.

BUG=v8:4280

Review-Url: https://codereview.chromium.org/2547043002
Cr-Commit-Position: refs/heads/master@{#41514}