Optimize BooleanTest with non-nullable input

Optimize BooleanTest with non-nullable input

The BooleanTest construct (IS [NOT] TRUE/FALSE/UNKNOWN) treats a NULL
input as the logical value "unknown".  However, when the input is
proven to be non-nullable, this special handling becomes redundant.
In such cases, the construct can be simplified directly to a boolean
expression or a constant.

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