pan/mdg: Scalarize 64-bit

pan/mdg: Scalarize 64-bit

We don't properly support 64-bit vec2 yet for various reasons, and as-is
vectorize will try to create vec4 which we choke on. Since any workloads
relying on 64-bit vector performance are already DOA at this point,
let's just do the conformant thing.

Fixes:

   dEQP-GLES31.functional.shaders.builtin_functions.integer.umulextended.uvec2_highp_compute

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6516>