Fix incorrect narrowing of nested array after assignment

[staabm]

closes phpstan/phpstan#13857
closes phpstan/phpstan#10089

[phpstan-bot]

This pull request has been marked as ready for review.

[VincentLanglet]

The condition

if ($i === 0) {
					$unionValues = true;
				} elseif (
					$overwritesExistingOffset === true
					&& $i === count($offsetTypes) - 1
					&&
						(
							$originalValueToWrite->isConstantScalarValue()->yes()
							|| !$offsetValueType->getIterableValueType()->isSuperTypeOf($valueToWrite)->yes()
						)
				) {
					$unionValues = true;
				}

is unclear to me @staabm I feel like a comment would be nice ; do you want to write one (since you worked on the PR) or let the bot generate one ?

[VincentLanglet]

The renaming makes the code clearer to me

[staabm]

I am 100% sure there is a better way to fix this.. but I spent already days on this (and related phpstan-bot PRs).
I will try to generalize and improve the logic in a future PR, but will merge it now as it still improves the status-quo