This ports the fix to #12919 to the normaliser. (#12919 was about
the flattener.) Because the fix is involved, this is done by
moving the critical piece of code to Coercion, and then calling
this from both the flattener and the normaliser.

The key bit is: simplifying type families in a type is always
a *homogeneous* operation. See #12919 for a discussion of why
this is the Right Way to simplify type families.

Also fixes #15549.

test case: dependent/should_compile/T14729{,kind}
           typecheck/should_compile/T15549[ab]