broadcom/compiler: handle vec2 load/store index

broadcom/compiler: handle vec2 load/store index

In vulkan, we load descriptors via vulkan resource index, which
returns a vec2, of which we want component 0 which holds the actual
index. Typically, this will be cleaned-up by the time we get to
emitting VIR so the index is a single scalar component, but there
are some cases where this might no be the case, so make sure we don't
assume it to be a scalar, like we do in other places.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19313>