1.0.14.36: faster PROPAGATE-FROM-SETS

1.0.14.36: faster PROPAGATE-FROM-SETS

 20-25% improvement for the test-case in bug 188.

 * New slot in LAMBDA-VAR: LAST-INITIAL-TYPE, which holds the last
   initial-type for that variable seen by PROPAGATE-FROM-SETS.

 * Be lazy, and don't PROPAGATE-TO-REFS unless something of
   interest has happened, to wit:

    -- One of the CSET nodes has a new, more specific type.

    -- INITIAL-TYPE has become more specific.

   This also allows us elide TYPE-UNION computation in the
   uninteresting cases.

 * Requires having NODE-REOPTIMIZE set when IR1-OPTIMIZE-SET
   is called.