multi-row INSERT broken with a sublink in a target column subscript

When looking at whether there exist other sites in the parser that are
like bug #19786, Claude found $subject.

CREATE TABLE ta (a int[]);
INSERT INTO ta (a[(SELECT 1)]) VALUES (1), (2);
ERROR: unexpected non-SELECT command in SubLink

The problem is that transformInsertStmt() transforms the target
columns' indirection once per VALUES row without copying it, while
transformSubLink() modifies the raw SubLink in place.

a[(1 IS NULL)::int + 1] fails too, like this:

INSERT INTO ta (a[(1 IS NULL)::int + 1]) VALUES (1), (2);
ERROR: unrecognized node type: 7

Attached patch fixes this.

--
Thanks, Amit Langote