Validator cfg fixes

Validator cfg fixes

- Find unreachable continue targets.  Look for back edges
  with a DFS traversal separate from the dominance traversals,
  where we count the OpLoopMerge from the header to the continue
  target as an edge in the graph.

- It's ok for a loop to have multiple back edges, provided
  they are all from the same block, and we call that the latch block.
  This may require a clarification/fix in the SPIR-V spec.

- Compute postdominance correctly for infinite loop:
  Bias *predecessor* traversal root finding so that you use
  a later block in the original list.  This ensures that
  for certain simple infinite loops in the CFG where neither
  block branches to a node without successors, that we'll
  compute the loop header as dominating the latch block, and the
  latch block as postdominating the loop header.