TypeSSA could delete exactness

We've previously discussed (e.g. in #8002) that TypeSSA no longer works well now that we automatically refine reference types to be exact wherever possible. The fix we had previously discussed was finishing and using the TypeGeneralizing pass (which might separately be good for code size by reducing the size of the type section) to remove exactness. But we could alternatively have a more crude fix where TypeSSA itself could delete exactness from as many types as possible. The only types for which it would not be able to erase exactness would be descriptor types that may flow into struct.new_desc instructions and any type that appears in an exact reference in a public heap type.

Start by reading the TypeSSA and TypeGeneralizing pass implementations and the prior discussion in #8002. Determine which approach is intended and verify how exactness is handled for descriptor types flowing to struct.new_desc and types used in exact references in public heap types; done means the selected approach has coverage for the remaining cases.