v3d/compiler: support swapping R/B channels in vertex attributes.

We will need this in Vulkan to support vertex format
VK_FORMAT_B8G8R8A8_UNORM.  The hardware doesn't allow to swizzle
vertex attribute components, so we need to do it in the shader.

v2:
 - Use nir_intrinsic_io_semantics() to retrieve the location instead
   of looping through the shader input variables (Eric).
 - Assert that we only have one component (Eric).

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com> (v1)
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>

src/broadcom/compiler/v3d_compiler.h

@@ -412,6 +412,12 @@ struct v3d_vs_key {
         struct v3d_varying_slot used_outputs[V3D_MAX_ANY_STAGE_INPUTS];
         uint8_t num_used_outputs;
 
+        /* A bit-mask indicating if we need to swap the R/B channels for
+         * vertex attributes. Since the hardware doesn't provide any
+         * means to swizzle vertex attributes we need to do it in the shader.
+         */
+        uint16_t va_swap_rb_mask;
+
         bool is_coord;
         bool per_vertex_point_size;
         bool clamp_color;