Fold the functionality of pull_up_reg() into evaluate_rec().
pull_up_reg() essentially returned a flag (EX_REG at the top, or not)
which can be represented with the outgoing flags word.
The check for recursion depth is made unneeded by doing the
"exceptional" case outside the recursion; luckily it is actually the
common case.
If the command line option yus to allow the underscore character in
symbols is NOT selected, then interpret underscore as a new binary
operator meaning to do a left shift of the left side value by the
number of bit positions indicated by the right side value. As for the
arithmetic operators, both values must be literal (numeric constants
or symbols equated to a numeric constant).
