Fix dominance calculation

Fix dominance calculation

Fixes dominance calculation when there is a forward arc from an
unreachable block A to a reachable block B.  Before this fix, we would
say that B is not dominated by the graph entry node, and instead say
that the immediate dominator of B is the psuedo-entry node of the
augmented CFG.

The fix:

- Dominance is defined in terms of a traversal from the entry block
  of the CFG.  So the forward DFS should start from the function
  entry block, not the pseudo-entry-block.

- When following edges backward during dominance calculations, only go to
  nodes that are actually reachable in the forward traversal.
  Important: the sense of reachability flips around when computing
  post-dominance.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/297