pan/midgard: Implement load/store_shared

pan/midgard: Implement load/store_shared

Shared memory is implemented almost identically to global memory from an
ISA perspective, so let's handle the new intrinsics. We include a code
path for constant offsets, which doesn't come up for globals.

Fixes dEQP-GLES31.functional.compute.basic.shared_var_single_invocation

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3775>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3775>