Strip PlaceHolderVars from index operands

Strip PlaceHolderVars from index operands

When pulling up a subquery, we may need to wrap its targetlist items
in PlaceHolderVars to enforce separate identity or as a result of
outer joins.  However, this causes any upper-level WHERE clauses
referencing these outputs to contain PlaceHolderVars, which prevents
indxpath.c from recognizing that they could be matched to index
columns or index expressions, potentially affecting the planner's
ability to use indexes.

To fix, explicitly strip PlaceHolderVars from index operands.  A
PlaceHolderVar appearing in a relation-scan-level expression is
effectively a no-op.  Nevertheless, to play it safe, we strip only
PlaceHolderVars that are not marked nullable.

The stripping is performed recursively to handle cases where
PlaceHolderVars are nested or interleaved with other node types.  To
minimize performance impact, we first use a lightweight walker to
check for the presence of strippable PlaceHolderVars.  The expensive
mutator is invoked only if a candidate is found, avoiding unnecessary
memory allocation and tree copying in the common case where no
PlaceHolderVars are present.

Back-patch to v18.  Although this issue exists before that, changes in
this version made it common enough to notice.  Given the lack of field
reports for older versions, I am not back-patching further.

Reported-by: Haowu Ge <gehaowu@bitmoe.com>
Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/62af586c-c270-44f3-9c5e-02c81d537e3d.gehaowu@bitmoe.com
Backpatch-through: 18