nir: Silence GCC maybe-uninitialized warnings.
nir: Silence GCC maybe-uninitialized warnings.

nir/nir_control_flow.c: In function ‘split_block_cursor.isra.11’:
nir/nir_control_flow.c:460:15: warning: ‘after’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       *_after = after;
               ^
nir/nir_control_flow.c:458:16: warning: ‘before’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       *_before = before;
                ^

Signed-off-by: Vinson Lee <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>
Reviewed-by: Connor Abbott <71178acffcc112b21e5858656e5751f5e4aa9364@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler