1.0.18.23: ALL calls with DX arguments should end their blocks

1.0.18.23: ALL calls with DX arguments should end their blocks

 * Consider this a clarification of 1.0.18.22: MAP-BLOCK-NLXES will
   miss calls that don't end their blocks.

   If let-conversion happens, as is the case most of the time, this is
   not an issue, since the LAMBDA-CALL-LEXENV will hold the lexenv
   with the cleanup.

   A :LET functional, however, should never appear in
   RECOGNIZE-DYNAMIC-EXTENT-LVARS, since let-conversion doesn't happen
   until later -- so the conditional was bogus.

   So: delete the conditional, and write a comment that points in
   the place where we depend on the block-endingness.

 * Add a FIXME note to another DX related place where we make nodes
   end their blocks, which I don't quite understand.