[stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

[stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

The StringCompareStub used to take its parameters on the (JavaScript)
stack, which made it impossible to use in TurboFan. Actually
StringCompareStub was currently completely unused. This changes the
calling convention to something TurboFan compatible and introduces a
CallInterfaceDescriptor for StringCompareStub. It also changes
HStringCompareAndBranch to use the StringCompareStub instead of using
the full blown CompareICStub for a stupid string comparison.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1347913003

Cr-Commit-Position: refs/heads/master@{#30818}