G1: interpreter post-barrier x86 code asserts index size of wrong buffer

The following assertion within G1BarrierSetAssembler::g1_write_barrier_post() in src/hotspot/cpu/x86/gc/g1/g1BarrierSetAssembler_x86.cpp:

// Generated code assumes that buffer index is pointer sized.
  STATIC_ASSERT(in_bytes(SATBMarkQueue::byte_width_of_index()) == sizeof(intptr_t));

refers to the size of the index of the SATBMarkQueue buffer, which is unused in the post-barrier. It should probably refer instead to G1DirtyCardQueue::byte_width_of_index().

This issue was found by [~kbarrett].