i965: Assign instead of compare inside assert

i965: Assign instead of compare inside assert

This is from a Coverity defect report.

src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
  1314  void
  1315  vec4_visitor::emit_block_move(dst_reg *dst, src_reg *src,
  1316                                const struct glsl_type *type, bool
predicated)
...
  1351     /* Do we need to worry about swizzling a swizzle? */
->1352     assert(src->swizzle = BRW_SWIZZLE_NOOP);
  1353     src->swizzle = swizzle_for_size(type->vector_elements);

Reported-by: Vinson Lee <vlee@vmware.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=40158
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>