Incorporate anti-dependencies into GCM

• Use anti-dependencies to affect GCM decisions.
  • Fix anti-dependency analysis.
  • Use new "write can't reach read without loops" condition.
  • Add utilities to Subgraph to compute the above condition.
• Add a more complicated anti-dependency test.
• Test 64-tiled matmul.

requested review from @aaronjc4

assigned to @rarbore2

merged

mentioned in commit 5f340d3f