use an IR1 attribute but allow arbitrary functions to determine if the
result is used or not.  This allows us to handle SORT and ADJUST-ARRAY
depending on the args.

Use 19c/boot-2005-11-1.lisp to bootstrap this change.

compiler/knownfun.lisp:
o Remove IMPORTANT-RESULT attribute.
o Add new slot to function-info the hold the function to indicate if
  the result is not used.
o Adjust %defknown with a new keyword arg :result-not-used for the new
  slot.
o Add functions for the result-not-used arg for
  o SORT-RESULT-NOT-USED-P: non-NIL if the sequence might be a list.
  o ADJUST-ARRAY-RESULT-NOT-USED-P: non-NIL if the array is not known
    to be adjustable.
  o FUNCTION-RESULT-NOT-USED-P:  non-NIL if function result must be
    used (for functions like nreverse).

compiler/ir1opt.lisp:
o Remove now unused function CHECK-IMPORTANT-RESULT
o Add check in IR1-OPTIMIZE-COMBINATION to see if function result
  should be used.

compiler/fndb.lisp:
o Remove IMPORTANT-RESULT attribute and replace with :result-not-used.