[GVNSink] Fix non-determinisms by using a deterministic ordering (#90995)

GVNSink used to order instructions based on their pointer values and was
prone to non-determinism because of that.
This patch ensures all the values stored are using a deterministic
order. I have also added a verfier(`ModelledPHI::verifyModelledPHI`) to
assert when ordering isn't preserved.

Additionally, I have added a test case (mirror graph image of an
existing test) that would have failed before this patch.

Fixes: #77852