i965/vec4: don't copy propagate misaligned registers

This means we would copy propagate partial reads or writes and that can affect
the result.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp

@@ -354,6 +354,9 @@ try_copy_propagate(const struct gen_device_info *devinfo,
       return false;
    }
 
+   if (inst->src[arg].offset % REG_SIZE || value.offset % REG_SIZE)
+      return false;
+
    bool has_source_modifiers = value.negate || value.abs;
 
    /* gen6 math and gen7+ SENDs from GRFs ignore source modifiers on