nir: Fix anonymous union initialization with older GCC.

nir: Fix anonymous union initialization with older GCC.

Fix this build error with GCC 4.4.7.

  CC     nir/nir_opt_copy_prop_vars.lo
nir/nir_opt_copy_prop_vars.c: In function ‘copy_prop_vars_block’:
nir/nir_opt_copy_prop_vars.c:765: error: unknown field ‘deref’ specified in initializer
nir/nir_opt_copy_prop_vars.c:765: warning: missing braces around initializer
nir/nir_opt_copy_prop_vars.c:765: warning: (near initialization for ‘(anonymous).<anonymous>’)
nir/nir_opt_copy_prop_vars.c:765: warning: initialization from incompatible pointer type

Fixes: 62332d139c8f ("nir: Add a local variable-based copy propagation pass")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>