C2: simplify transfer of GC barrier data from Ideal to Mach nodes

Slightly simplify Matcher::ReduceInst() by replacing the following lines [1]:

  if (leaf->is_LoadStore()) {
    mach->set_barrier_data(leaf->as_LoadStore()->barrier_data());
  } else if (leaf->is_Mem()) {
    mach->set_barrier_data(leaf->as_Mem()->barrier_data());
  }

with a call to MemNode::barrier_data(), added recently by JDK-8322692.

[1] https://github.com/openjdk/jdk/blob/3f1d9c441ea98910d9483e133bccfac784db393d/src/hotspot/share/opto/matcher.cpp#L1892-L1897