Fix bug in selection subexpression handling.

Did not work correctly if
1. a static expression was passed to a SPAR_ATOMVAL parameter,
2. other parameters to the same selection method were dynamic
   (so that the expression would not be completely eliminated during
   compilation), and
3. the evaluation group of the SEL_EXPRESSION element was dynamic
   (so that the SPAR_ATOMVAL expression could be evaluated for a
   different group during evaluation and compilation).
In this case, the static atom-valued parameter got evaluated during
compilation for the maximal evaluation group and replaced by a constant.
During evaluation, if the evaluation group was smaller, the values for
that expression were no longer correct.
See #1216 for what kinds of concrete selections this applies to.

Subexpression handling in the selections is starting to (again) get very
messy, but that is a topic for a larger-scale refactoring...

Fixes #1216.

Change-Id: Ic6b7f9b8df661a9c78d7862b981a07e65a7ebdbf
Merge "Fix bug in selection subexpression handling."