avoid emitting degenerate aliases in peval

avoid emitting degenerate aliases in peval

* module/language/tree-il/peval.scm (<operand>, make-operand)
  (make-bound-operands, peval): Avoid emitting needless aliases in
  degenerate cases of let.
  (visit-operand): If we visit an operand with a fresh counter and have
  to abort, record that fact.

* test-suite/tests/peval.test ("partial evaluation"): Add a test.