[developer] KeymanWeb compiler needs to optimise static stores

When compiling a keyboard with repeated platform statements, this generates many repeated stores. For an example, see https://github.com/silcam/Cameroon-Keyboard/blob/BrokenRules/src/km/cameroon_qwerty/source/cameroon_qwerty.kmn

If repeated stores are not used as a variable store, then they should be optimised to a single reference at compile time.