opt: placeholder fast path should respect index hints

[mgartner]

The placeholder fast-path, used for simple prepared statements, does not consider index hints when choosing an index for the placeholder scan. It may pick an index that was not forced. This causes an error when the query is executed because the optimizer did not find a plan that uses the forced index.

For example:

CREATE TABLE t (
  a INT,
  b INT,
  c INT,
  PRIMARY KEY (a, b),
  INDEX i (a, b)
);

SELECT * FROM t@i WHERE a = 11 AND b = 22;
--  a | b | c
-- ----+---+----
-- (0 rows)

PREPARE p AS
SELECT * FROM t@i WHERE a = $1 AND b = $2;

EXECUTE p(11, 22);
-- ERROR: index "i" cannot be used for this query
-- SQLSTATE: 42809

This bug is present since the placeholder fast path was introduced in v21.2.0.

Jira issue: CRDB-51019