c++: small tweak for cp_fold_r

This patch is an optimization tweak for cp_fold_r.  If we cp_fold_r the
COND_EXPR's op0 first, we may be able to evaluate it to a constant if -O.
cp_fold has:

3143         if (callee && DECL_DECLARED_CONSTEXPR_P (callee)
3144             && !flag_no_inline)
...
3151             r = maybe_constant_value (x, /*decl=*/NULL_TREE,

flag_no_inline is 1 for -O0:

1124   if (opts->x_optimize == 0)
1125     {
1126       /* Inlining does not work if not optimizing,
1127          so force it not to be done.  */
1128       opts->x_warn_inline = 0;
1129       opts->x_flag_no_inline = 1;
1130     }

but otherwise it's 0 and cp_fold will maybe_constant_value calls to
constexpr functions.  And if it doesn't, then folding the COND_EXPR
will keep both arms, and we can avoid calling maybe_constant_value.

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold_r): Don't call maybe_constant_value.