Detect pattern equivalence after constexpr evaluation

We're about to introduce a (user requested) check for a syntactically redundant pattern (which indicates the user made a mistake, since the second syntactically-identical pattern match would never match, only the first). However, we could do better if we could compare patterns for equivalence after constexprs are resolved, would require a "are these patterns definitely matching identical things" helper, which we don't currently have. Right now this will be an opportunistic check we can implement quickly for some degree of checking help.