The main point is described in Note [Solve order for RULES].
I'm not sure if the potential bug described there could actually
happen, but I bet it could.  Anyway, this patch explicitly solves
LHS constraints and *then* RHS constraints (see the Note).

I also moved simplifyRule from TcSimplify (a large module) to
TcRules (a small one), which brings related code together.
It did mean I had to export runTcS from TcSimplify, but I think
that's a price worth paying.