[flang] Procedure distinguishability rules for operators

Generic operators and assignment have different rules than generic
names for their procedures being distinguishable.

Implement those rules in `DistinguishableOpOrAssign`. The rules are
considerably simpler: they must have the name number of dummy arguments
and at least one pair in the same position must be distinguishable.

Fixes issue flang-compiler/f18#563.

Original-commit: flang-compiler/f18@276bb082060dcbdb7d2b2a7dbe344bb72ecddab6
Reviewed-on: https://github.com/flang-compiler/f18/pull/576