Adjust assertion when casting to an unregistered operation

This assertion does not achieve what it meant to do originally, as it
would fire only when applied to an unregistered operation, which is a
fairly rare circumstance (it needs a dialect or context allowing
unregistered operation in the input in the first place).
Instead we relax it to only fire when it should have matched but didn't
because of the misconfiguration.

Differential Revision: https://reviews.llvm.org/D86588