tgsi: provide a way to encode memory qualifiers for SSBO

tgsi: provide a way to encode memory qualifiers for SSBO

Each load/store on most hardware can specify what caching to do. Since
SSBO allows individual variables to also have separate caching modes,
allow loads/stores to have the qualifiers instead of attempting to
encode them in declarations.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>