Bug: Fix sort_merge_join hint crash when used at query block level

[Yangxingyu3028]

Description:
SORT_MERGE_JOIN hint was incorrectly registered as a non-QB-level,
non-switch hint (false, false in opt_hint_info), and was also
incorrectly treated as a compound hint in is_compound_hint() and
get_complex_hints(). When SORT_MERGE_JOIN was used at query block
level (e.g., /+sort_merge_join()/), calling get_complex_hints()
would crash because the hint type was routed to the compound hint
path (via Opt_hints_table::get_compound_key_hint()), which
returned an invalid pointer for SORT_MERGE_JOIN_HINT_ENUM.

How to repeat:

create table t1 (c int, b int);
create table t2 (a int, b int);
create table t3 (b int, c int);
insert into t1 values (10,1);
insert into t1 values (3 ,1);
insert into t1 values (3 ,2);
insert into t2 values (2, 1);
insert into t2 values (3, 1);
insert into t3 values (1, 3);
insert into t3 values (1,10);
ANALYZE TABLE t1, t2, t3;

select /*+sort_merge_join()*/ *
from t1
inner join t2 on t1.b=t2.b
inner join t3 on t3.c = t1.c and t3.b = t2.b;