Restore `Opaque` behavior to coherence check
Fixes#99663.
This broke in 84c3fcd2a0285c06a682c9b064640084e4c7271b. I'm not exactly certain that adding this behavior back is necessarily correct, but at least the UI test I provided may stimulate some thoughts.
I think delaying a bug here is certainly not correct in the case of opaques -- if we want to change coherence behavior for opaques, then we should at least be emitting a new error.
r? ``@lcnr``
99350de0d5