Handle access chain with no index in SROA. (#2304)

It is legal, but not generated by any SPIR-V producer: an OpAccessChain
with no indexes.  This is essentially just a copy of the pointer.

I have decided to treat it like an OpCopyObject.  In CheckUses, we
return that it is not okay.

When looking at this I realized that we had code in GetUsedComponents
that cannot be reached.  If there is a use in an OpCopyObject the it
will not call GetUsedComponents.  I removed that dead code.

Fixes https://crbug.com/918311.