Rollup merge of #124027 - oli-obk:define_opaque_types9, r=compiler-er…

…rors

Prefer identity equality over equating types during coercion.

These types are always generic only over their own generic parameters with no inference variables involved.

r? `@compiler-errors`

I love touching code that [hasn't changed meaningfully since 2016](rust-lang/rust#41937)