The types btr_latch_mode and mtr_memo_type_t are partly derived from
rw_lock_type_t. Despite that, some code for converting between them
is using conditions instead of bitwise arithmetics.

Let us define btr_latch_mode in such a way that more conversions to
rw_lock_type_t are possible by bitwise and.

Some SPATIAL INDEX code that assumed !(BTR_MODIFY_TREE & BTR_MODIFY_LEAF)
was adjusted.