Better sharing of join points: #19996

This patch fixes #19996 (closed) and #20049 (closed).

• In the StrictArg case of mkDupableContWithDmds, don't use Plan A for data constructors

• In postInlineUnconditionally, don't inline JoinIds. Avoids inlining

  join $j x = Just x
  in case blah of
     A -> $j x1
     B -> $j x2
     C -> $j x3

• In mkDupableStrictBind and mkDupableStrictAlt, create join points (much) more often: exprIsTrivial rather than exprIsDupable. This is probably too much, but we'll see