1.0.29.3: better reporting for failure to stack allocate

1.0.29.3: better reporting for failure to stack allocate

 * If the LVAR has no uses left, it is not good for DX.

 * When flushing the destination of a DX lvar, note that its uses will
   not be stack allocated.

 * Pull out the failure to stack allocate reporting into
   NOTE-NO-STACK-ALLOCATION, relax the filter to allow complaints
   about non-constant REFs, and omit notes for flushed and flushable
   combinations. TODO: the compiler should know about non-consing
   functions, so that it can avoid inane notes like "could not stack
   allocate the result of (CAR X)" should someone declare that DX.

 * Muffle compiler notes from WITH-PINNED-OBJECTS, since our paranoid
   use of DX-LET there is liable to cause lots of confusing "unable to
   stack allocate" notes.