don't emit redundant error when checking a named type against itself

[orklah]

#6499 introduced a new regression indirectly. Psalm is now aware that $x instanceof $y means that $x is an $y, but when $x and $y have the same type, it doesn't mean the check is redundant.

For example: ErrorException and TypeError can both be represented under throwable but $x instanceof $y would return false

This will fix #6736

[weirdan]

I think this needs a test.

[orklah]

Indeed!