Rollup merge of #107074 - lcnr:validate-dont-skip-opaque, r=compiler-errors

remove unnecessary check for opaque types

this isn't needed and may hide some errors.

after analysis there are no opaque types so it's a noop anyways

before analysis there are opaque types but due to `Reveal::UserFacing` we don't reveal them. `is_subtype` simply discards the opaque type constraints as these will get checked again during mir borrowck.

r? types

want to land this after the beta-cutoff as mir validator changes are apparently pretty scary