18.104.22.168: multiply-used DX LVARS
* HANDLE-NESTED-DYNAMIC-EXTENT maps over all the uses of the LVAR,
collecting their argument lvars as well.
* RECHECK-DYNAMIC-EXTENT-LVARS accepts multiply-used DX LVARs,
checking that all uses support stack allocation.
* UPDATE-UVL-LIVE-SETS accepts multiply-used DX LVARs, doing that
lifetime merging with all uses.
* ...and OOPS, move the NEWS entries of .5 and .6 to a new section for