Some conditions around btr_latch_mode could be eliminated

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.

If we align btr_latch_mode better with mtr_memo_type_t, some conversions by bitwise arithmetics will become possible. For example, the arithmetic expression

	const rw_lock_type_t mode = rw_lock_type_t(

		latch_mode & (RW_X_LATCH | RW_S_LATCH));

would replace conditional code like the following:

	mode = latch_mode == BTR_MODIFY_LEAF ? RW_X_LATCH : RW_S_LATCH;