Variable<> was redundant after LValue became a template class. We can
recycle it as a container for Values.

Bug swiftshader:27

Change-Id: Ic5ed6f3f7e26579fd38f0e809f8bada1e3639d00
Reviewed-on: https://swiftshader-review.googlesource.com/8071Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>