bug: ALTER TABLE ADD VIRTUAL GENERATED COLUMN with table rewrite

hi.

bug demo:
CREATE TABLE gtest20a (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a
* 2) VIRTUAL);
ALTER TABLE gtest20a ADD COLUMN c float8 DEFAULT RANDOM() CHECK (b < 60);
ERROR: no generation expression found for column number 2 of table
"pg_temp_17306"

issue is that
in ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap)
we need use existing (tab->relid) relation get the generated
expression, not use OIDNewHeap,
since we don't populate OIDNewHeap related generated expressions.

The attached patch fixes this issue.