Handle non-constant accesses in memory objects (copy prop arrays)

The first implementation of MemroyObject, which is used in copy
propagate arrays, forced the access chain to be like the access chains
in OpCompositeExtract.  This excluded the possibility of the memory
object from representing an array element that was extracted with a
variable index.   Looking at the code, that restriction is not
neccessary.  I also see some opportunities for doing this in some real
shaders.

Contributes to #1430.