Do ScalarArrayOp estimation correctly when array is a stable expression.

Most estimation functions apply estimate_expression_value to see if they
can reduce an expression to a constant; the key difference is that it
allows evaluation of stable as well as immutable functions in hopes of
ending up with a simple Const node.  scalararraysel didn't get the memo
though, and neither did gincost_opexpr/gincost_scalararrayopexpr.  Fix
that, and remove a now-unnecessary estimate_expression_value step in the
subsidiary function scalararraysel_containment.

Per complaint from Alexey Klyukin.  Back-patch to 9.3.  The problem
goes back further, but I'm hesitant to change estimation behavior in
long-stable release branches.