Assert and record that code generation requires distinct uiques for let-binders

Simon writes in #15754:

Another invariant that the code generator needs is (I believe) that every let-binder has a distinct unique. We can't re-use the same unique, even in a different scope, let alone shadowing. Why? Because (I believe) that the code generator uses these uniques to generate unique top level labels for the entry code and info table for the closure.

Now, it's probably the case that they are unique anyway; but we should either

1. Write this down as a invariant of STG, or
2. Establish this invariant in the immediately-before-codegen pass that gets the free vars right, or
3. Not assume it in the code generator

mentioned in issue #15754 (closed)

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I have no strong opinion about 1-3.

This is currently done by CorePrep, and noted explicitly in the comments (CorePrep.hs):

6.  Clone all local Ids.
    This means that all such Ids are unique, rather than the
    weaker guarantee of no clashes which the simplifier provides.
    And that is what the code generator needs.

    We don't clone TyVars or CoVars. The code gen doesn't need that,
    and doing so would be tiresome because then we'd need
    to substitute in types and coercions.

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