[Variant 2 of the fix: collect the attached conditions]

Problem:
make_join_select() has a section of code which starts with
 "We plan to scan all rows. Check again if we should use an index."

the code in that section will [unnecessarily] re-run the range
optimizer using this condition:

  condition_attached_to_current_table AND current_table's_ON_expr

Note that the original invocation of range optimizer in
make_join_statistics was done using the whole select's WHERE condition.
Taking the whole select's WHERE condition and using multiple-equalities
allowed the range optimizer to infer more range restrictions.

The fix:
- Do range optimization using a condition that is an AND of this table's
condition and all of the previous tables' conditions.
- Also, fix the range optimizer to prefer SEL_ARGs with type=KEY_RANGE
over SEL_ARGS with type=MAYBE_KEY, regardless of the key part.
Computing
key_and(
  SEL_ARG(type=MAYBE_KEY key_part=1),
  SEL_ARG(type=KEY_RANGE, key_part=2)
)
will now produce the SEL_ARG with type=KEY_RANGE.