Make more use of castNode()

src/backend/commands/copy.c

src/backend/nodes/nodeFuncs.c

src/backend/optimizer/path/allpaths.c

src/backend/optimizer/path/costsize.c

src/backend/optimizer/path/indxpath.c

src/backend/optimizer/path/joinrels.c

src/backend/optimizer/plan/analyzejoins.c

src/backend/optimizer/plan/createplan.c

src/backend/optimizer/plan/planner.c

src/backend/optimizer/plan/setrefs.c

src/backend/optimizer/plan/subselect.c

src/backend/optimizer/prep/prepjointree.c

src/backend/optimizer/prep/prepunion.c

src/backend/optimizer/util/clauses.c

src/backend/optimizer/util/orclauses.c

src/backend/optimizer/util/restrictinfo.c

src/backend/parser/analyze.c

src/backend/parser/gram.y

src/backend/parser/parse_clause.c

src/backend/parser/parse_collate.c