Teach planner to transform "x IS [NOT] DISTINCT FROM NULL" to a NullTest

Teach planner to transform "x IS [NOT] DISTINCT FROM NULL" to a NullTest

In the spirit of 8d19d0e13, this patch teaches the planner about the
principle that NullTest with !argisrow is fully equivalent to SQL's IS
[NOT] DISTINCT FROM NULL.

The parser already performs this transformation for literal NULLs.
However, a DistinctExpr expression with one input evaluating to NULL
during planning (e.g., via const-folding of "1 + NULL" or parameter
substitution in custom plans) currently remains as a DistinctExpr
node.

This patch closes the gap for const-folded NULLs.  It specifically
targets the case where one input is a constant NULL and the other is a
nullable non-constant expression.  (If the other input were otherwise,
the DistinctExpr node would have already been simplified to a constant
TRUE or FALSE.)

This transformation can be beneficial because NullTest is much more
amenable to optimization than DistinctExpr, since the planner knows a
good deal about the former and next to nothing about the latter.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAMbWs49BMAOWvkdSHxpUDnniqJcEcGq3_8dd_5wTR4xrQY8urA@mail.gmail.com