Fix ConstantArrayType optional key unsetting via removeLastElements

[herndlm]

Closes phpstan/phpstan#7351

That was a really interesting one. The changes in #1352 were triggering it, but that bug was already there and I just seem to have adopted it too.

The problem was that while traversing the array and using $i as counter variable, we cannot simply unset the optional key on position $i if the array element on that position is going to be removed.
E.g. the array array{a: 0, b?: 1} would be represented with the $optionalKeys array{0: 1} and if if we remove b from the original array (which is on position 1) then we don't remove anything from $optionalKeys. Instead we have to unset the key of the value 1 (which is actually 0) to keep the state consistent.

Without this change we ended up with an array with one entry but 2 of them being marked optional and this triggered the fatal error later via ConstantArrayType::getAllArrays

[ondrejmirtes]

Yes, in optionalKeys the value is significant, not the index :)

[ondrejmirtes]

Thank you!