_getSelectDict was also calling itself on sub-sql_expressions, but this
was useless: they would already have called it, and discarded one of its
returned values. Instead, preserve both returned values in __init__ and
access these attributes directly when iterating over sub-sql_expressions.
Also, fold canMergeSelectDict back into its only 2 call places, saving one
getattr and one call.
Also, get rid of a few properties which were only used in _getSelectDict.
Also, get rid of two iterable copies: sql_expression_list when it is
already a list (no need to cast it again to a list, we just built one with
the list comprehension), and select_dict being cast into a list when
entering _getSelectDict, despite not being used (besides in extra
_getSelectDict calls, which are now entirely avoided).