simplNonRecJoinPoint: Handle Shadowing correctly

Previously, (since 33452df), simplNonRecJoinPoint would do the wrong
thing in the presence of shadowing: It analyzed the RHS of a join
binding with the environment for the body. In particular, with

    foo x =
      join x = x * x
      in x

where there is shadowing, it renames the inner x to x1, and should
produce

    foo x =
      join x1 = x * x
      in x1

but because the substitution (x ↦ x1) is also used on the RHS we get the
bogus

    foo x =
      join x1 = x1 * x1
      in x1

Fixed this by adding a `rhs_se` parameter, analogous to `simplNonRecE`
and `simplLazyBind`.

Differential Revision: https://phabricator.haskell.org/D4130

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