spirv: Don't emit RMW for vector indexing in shared or global
Anything that fails the is_external_block check is getting the vtn_local_load/store path which does read-modify-write which isn't correct if the variable mode can be written cross-workgroup. Cc: mesa-stable@lists.freedesktop.org Reviewed-by:Jesse Natalie <jenatali@microsoft.com> Part-of: <mesa/mesa!6372>