[turbofan] Optimize strict equality of unique values.

[turbofan] Optimize strict equality of unique values.

If both inputs to JSStrictEqual/JSStrictNotEqual are unique values (i.e.
values with a canonical representation), we can lower the comparison to
ReferenceEqual instead of StringEqual or CompareIC.

Review URL: https://codereview.chromium.org/1154303002

Cr-Commit-Position: refs/heads/master@{#28646}