Handle nodes that may appear in GraphPattern expression trees

Handle nodes that may appear in GraphPattern expression trees

expression_tree_mutator_impl() did not handle T_GraphPattern,
T_GraphElementPattern, and T_GraphPropertyRef.  The corresponding
expression_tree_walker_impl() already handles all three node types.
This causes an "unrecognized node type" error whenever a GRAPH_TABLE
appeared in an expression tree.

While at it, also update raw_expression_tree_walker() and
expression_tree_walker() to handle missing nodes that may appear in
GraphPattern expression trees.  When raw_expression_tree_walker() is
called, GraphElementPattern::labelexpr contains ColumnRefs instead of
GraphLabelRefs.  Hence those are not handled in
raw_expression_tree_walker().

Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAHg%2BQDc97WFTSkXg%3Dg_ZAH8GnY2gJrvq72cs%2BYjqEAuZgXnkAQ%40mail.gmail.com