This fixes a regression for rails#35864.

Usually, stashed joins (mainly eager loading) are performed as LEFT
JOINs.
But the case of merging joins/left_joins of different class, that
(stashed) joins are performed as the same `join_type` as the parent
context for now.
Since rails#35864, both (joins/left_joins) stashed joins might be contained
in `joins_values`, so each stashed joins should maintain its own
`join_type` context.

Fixes rails#36103.

…joins

rails#36293 was an issue for through association with `joins` for a long
time, but since rails#35864 through association with `left_joins` would also
be affected by the issue.

Implicit through table joins should be appeared before user supplied
joins, otherwise loading through association with joins will cause a
statement invalid error.

Fixes rails#36293.

```
% ARCONN=postgresql bundle exec ruby -w -Itest test/cases/associations/has_many_through_associations_test
.rb -n test_through_association_with_joins
Using postgresql
Run options: -n test_through_association_with_joins --seed 7116

# Running:

E

Error:
HasManyThroughAssociationsTest#test_through_association_with_joins:
ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  missing FROM-clause entry for table "posts"
LINE 1: ... "comments_posts" ON "comments_posts"."post_id" = "posts"."i...
                                                             ^
: SELECT "comments".* FROM "comments" INNER JOIN "comments" "comments_posts" ON "comments_posts"."post_id" = "posts"."id" INNER JOIN "posts" ON "comments"."post_id" = "posts"."id" WHERE "posts"."author_id" = $1

rails test test/cases/associations/has_many_through_associations_test.rb:61

Finished in 0.388657s, 2.5730 runs/s, 0.0000 assertions/s.
1 runs, 0 assertions, 0 failures, 1 errors, 0 skips
```

…associations

rails#38597 is caused by rails#35864.

To reproduce this issue, at least it is required four different models
and three left joins from different relations.

When merging a relation from different model, new stashed (left) joins
should be placed before existing stashed joins, but rails#35864 had broken
that expectation if left joins are stashed multiple times.

This fixes that stashed left joins order as expected.

Fixes rails#38597.