peval: don't propagate expressions that access memory

peval: don't propagate expressions that access memory

* module/language/tree-il/optimize.scm (peval): Rename
  `pure-expression?' to `constant-expression?', in the sense of GCC's
  `pure' and `const'.  A <toplevel-ref> is not constant, because it can
  be mutated.  A <dynref> isn't constant either, for the same reason.

* test-suite/tests/tree-il.test ("partial evaluation"): Add a test, and
  update existing tests that assumed that toplevel-ref would propagate.