nir: Drop the unused instr arg for src/dest copy functions.

Now that we don't use ralloc, we don't need this arg to get at the right
ralloc ctx.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11776>

src/compiler/nir/nir_search.c

@@ -524,8 +524,7 @@ construct_value(nir_builder *build,
       assert(state->variables_seen & (1 << var->variable));
 
       nir_alu_src val = { NIR_SRC_INIT };
-      nir_alu_src_copy(&val, &state->variables[var->variable],
-                       (void *)build->shader);
+      nir_alu_src_copy(&val, &state->variables[var->variable]);
       assert(!var->is_constant);
 
       for (unsigned i = 0; i < NIR_MAX_VEC_COMPONENTS; i++)