SROA: Only create symbols that are loaded.

Currently in scalar replacement, we create a new variable for every
memeber of the composite being divided.  It is often overkill, because
not all of those members will be used.  This change will check which
elements are used and only create variable for the members that are
used.

This reduces the compile time for one set of shader from 248s to 165s.

Part of https://github.com/KhronosGroup/SPIRV-Tools/issues/1494.