Remove automatic type detection when emitting a select

[dcodeIO]

Fixes #2073 by removing error-prone automatic type detection when emitting a select expression. Was a legacy option anyway, from a time where selects didn't specify a type.

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[MaxGraey]

You removed automatic type detection for select op. And that's what caused those problems?

[dcodeIO]

Yes, the issue came from the attempt of automatic detection when one arm was of type unreachable, then not matching for both arms, like in the closure case where one of the variables is in another scope that currently yields a diagnostic. The fix now doesn't rely on this detection anymore, but instead the expected type is explicitly specified, in turn rendering the assertion unnecessary.

[MaxGraey]

I see. LGTM