I incorrectly (9a16248d) added an UNIMPLEMENTED to variables that have a storage type that is neither Input or Output.  However, other storage types do actually work.

Revert that change.

Change-Id: I5ec36e9d6d536dcdd085c93e9f00782310e118d4
Reviewed-on: https://swiftshader-review.googlesource.com/c/25848Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>