spirv/nir: Fix the stream ID when emitting a primitive or vertex

It looks like it was previously taking the SPIR-V instruction number
directly instead of looking up the constant value.

v2: use vtn_constant_value helper (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/spirv/spirv_to_nir.c

@@ -3218,9 +3218,12 @@ vtn_handle_barrier(struct vtn_builder *b, SpvOp opcode,
 
       switch (opcode) {
       case SpvOpEmitStreamVertex:
-      case SpvOpEndStreamPrimitive:
-         nir_intrinsic_set_stream_id(intrin, w[1]);
+      case SpvOpEndStreamPrimitive: {
+         unsigned stream = vtn_constant_value(b, w[1])->values[0].u32[0];
+         nir_intrinsic_set_stream_id(intrin, stream);
          break;
+      }
+
       default:
          break;
       }