Disallow calls on the RHS of a :(=) expr in lowered IR

[Keno]

Since lowered IR is not often consumed by Cassette-like tools for code transformation, I think it could use a bit of clean up. One thing that has bothered me for a bit is code that looks like the following.

stmt = expr = stmts[i]
if isexpr(stmt, :(=))
    expr = stmt.args[2]
end

# Do something with expr
expr = Expr(:call, :something_fun, expr.args[2:end]...)

if isexpr(stmt, :(=))
   stmt.args[2] = expr
end

I think it would make sense to only allow SSAValues on the RHS of an :(=) Expr in lowered IR, so code that e.g. only wants to rewrite all the calls in a function can simply ignore them.

[JeffBezanson]

This would be nice. I think we probably tried it at one point, and it made the IR significantly larger, which is not surprising. With a more compact encoding it might be ok.