avoid "if ... else if ... else" logic in ada-lang.c::ada_evaluate_subexp

avoid "if ... else if ... else" logic in ada-lang.c::ada_evaluate_subexp

The OP_VAR_VALUE branch in ada_evaluate_subexp is written with
multiple "if ... else if ... else if ... else ..." block. But
in practice, these blocks  all either goto out of that block of
code, or return.

This patch rewrites this code slightly by replacing the "else if"-s
by simple "if"s. This should better reflect the ideal processing
where we try to do a standard eval whenever possible, and only
do something else when the standard eval does not work. On a pratical
level, this patch makes it easier to fall through to the default
processing when none of the special situations are detected, thus
making it easier to add more handlers of those special situations;
or to remove them as they no longer become necessary!

gdb/ChangeLog:

        * ada-lang.c (ada_evaluate_subexp) <OP_VAR_VALUE>: Slight code
        rewrite to avoid "else if" and "else" constructs.  Should be
        a no-op in practice.