Remove IssetCheck/MutatingScope::issetCheck deviations and remove too early exits

[herndlm]

While trying to check the deviations in the code that handles ArrayDimFetch I noticed the following things that I re-did in MutatingScope::issetCheck() because they were done at some point in IssetCheck

• the maybe block was removed
• Scope::isSpecified was added additionally to the yes block
• the early exit in the yes block was removed (that also triggered errors when I used it via Improve isset specification in falsy scope #1781)

Additionally, I thought the early exit in the no block looked weird too and removed it as well since tests were green.

I hope this helps in removing duplications and I wanted to see if CI is also green..

[herndlm]

Fyi I have a rough plan about re-usage of this code already that I'll be working on in the evening :)

[ondrejmirtes]

Thank you!