Fix erroneous uses of the type manager in copy-prop-arrays. (#1942)

There are a few spots where copy propagate arrays is trying
to go from a Type to an id, but the type is not unique.  When generating
code this pass needs specific ids, otherwise we get type mismatches.
However, the ambigous types means we can sometimes get the wrong type
and generate invalid code.

That code has been rewritten to not rely on the type manager, and just
look at the instructions instead.

I have opened https://github.com/KhronosGroup/SPIRV-Tools/issues/1939 to
try to get a way to make this more robust.