[cleanup] Simplify CodeStubAssembler::Equals

Restructuring the code a bit, dropping unnecessary labels and
trivial comments. Replacing the InstanceType jump table with
a few comparisons is faster and smaller.

No change in functionality intended.

Bug: v8:6921
Change-Id: Ia8c751717c0dbcd6a664ca508ddeff898cd84359
Reviewed-on: https://chromium-review.googlesource.com/729466
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48769}