MDEV-37913: disable_index_merge_plans causes SELECT data loss when more than 100 ORs

[spetrunia]

• The Jira issue number for this PR is: MDEV-37913

Description

MDEV-37913: disable_index_merge_plans causes SELECT data loss when more than 100 ORs

(Variant 2)
SEL_TREE* tree_or(SEL_TREE *X, SEL_TREE *Y) tries to conserve memory by
reusing object *X for the return value when possible.

MDEV-34620 has added logic to disable construction of index_merge plans
for N-way ORs with large N. That logic interfered with object reuse logic:

for the parameters of:
X = SEL_TREE{ trees=[key1_treeA, key2_treeB]}
Y = SEL_TREE{ trees=[key1_treeC]}

we would decide to reuse object X.
For key1, we would produce key_or(key1_treeA, key1_treeC)
but key2_treeB would be just left there.
Then, we would construct a range scan from key2_treeB.

Fixed by moving the "disable building index_merge plans" logic into a
location where it would not interfere with object reuse logic.

• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

[DaveGosselin-MariaDB]

Verified that new return codepath is executed by new test case.
Verified that the tests from related MDEV-34620 pass and exercise the new return codepath.
Verified that the test fails without the fix because the prior logic could force tree1's key map to be empty incorrectly.
Checked code coverage results.
Ran complete regression suite.