luci-base: cbi.js: fix handling of inverse-depends, combobox validation

 - Properly handle logic for evaluating inverted dependencies means
   when depending on another field *not* having a given value

 - Perform datatype validation *after* populating combobox choices to
   avoid triggering a false-positive when validating an .rmempty = false
   combobox during form instantiation

Signed-off-by: Jo-Philipp Wich <jo@mein.io>