Generic properties are not properly refined by is_array checks in local usage

[stof]

Bug report

When a variable contains a generic type for which the bound is a union involving string|array<string>, a is_array check does not properly refine the property as being a string in the else branch.

Code snippet that reproduces the problem

https://phpstan.org/r/f4025a6a-f221-436c-804b-ce7d15d9cab3

Expected output

No error should be reported.

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[ondrejmirtes]

Fixed: phpstan/phpstan-src#2652

[thg2k]

Simplified test case:

https://phpstan.org/r/bc6b8adf-a25e-4960-8f43-eca8399cf509

[github-actions]

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