Make record selection look cheap

exprIsCheap was already making record selection look cheap, esp for
dictionary selection; e.g.
   (==) dEq

The idea is to allow those record selections to migrate inside a
lambda, because on the whole it's more expensive to share than
repeat them.

But sometimes dictionary selection is nested; e.g.
   (==) ($p1Num dNum)

This turned out to make a (fragile) rewrite rule fire a little
more robustly, but it seems like a good idea in any case.

This patch makes nested record selections also look cheap.