spirv: replace assert() with unreachable()

This fixes an uninitialized warning for is_vertex_input.

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

src/compiler/spirv/vtn_variables.c

@@ -1054,7 +1054,7 @@ var_decoration_cb(struct vtn_builder *b, struct vtn_value *val, int member,
          is_vertex_input = false;
          location += VARYING_SLOT_VAR0;
       } else {
-         assert(!"Location must be on input or output variable");
+         unreachable("Location must be on input or output variable");
       }
 
       if (vtn_var->var) {