Add dynamic_prop::is_constant

I noticed many spots checking whether a dynamic property's kind is
PROP_CONST.  Some spots, I think, are doing a slightly incorrect check
-- checking for != PROP_UNDEFINED where == PROP_CONST is actually
required, the key thing being that const_val may only be called for
PROP_CONST properties.

This patch adds dynamic::is_constant and then updates these checks to
use it.

Regression tested on x86-64 Fedora 36.