Remove mic-drop hack from if analysis

[muglug]

Handling of if conditions had a path I called "mic drop" that dealt with the following code

if (<expr>) {
  <some code>
  return;
} // no elseif or else

While a useful shortcut, this created a bunch of special cases that don't work if an empty else {} is appended to the if condition — many examples in tests/TypeReconciliation/TypeAlgebraTest.php fail when that empty else {} is used.

This PR contains the necessary fixes to make most everything work as expected without the mic drop hack.

[muglug]

The endtoend test failure with PHPUnit is, I think, a true positive. I'll update the baseline in that repo.

[orklah]

It makes $a mixed here?

It seemed correctly inferred to hello before

[muglug]

Yeah, unfortunately that was dodgy too: https://psalm.dev/r/a1a5253dc3. Since assignments in conditionals are a known pain, I thought this was a reasonable compromise (there are many assignment in conditional tests, and this was the only one affected).

[orklah]

Seems reasonable

[orklah]

Seems great!

While you're in this code, I'd be grateful if you could leave a little comment to explain the role of the different scope and contexts objects for analyzing the if().

I tried to check this code a few times but I get lost with the if_context, outer_scope, post_if_context etc...

[orklah]

Thanks!