Add loop descriptors and some required dominator tree extensions.

Add loop descriptors and some required dominator tree extensions.

Add post-order tree iterator.

Add DominatorTreeNode extensions:
 - Add begin/end methods to do pre-order and post-order tree traversal from a given DominatorTreeNode

Add DominatorTree extensions:
  - Add begin/end methods to do pre-order and post-order tree traversal
  - Tree traversal ignore by default the pseudo entry block
  - Retrieve a DominatorTreeNode from a basic block

Add loop descriptor:
  - Add a LoopDescriptor class to register all loops in a given function.
  - Add a Loop class to describe a loop:
    - Loop parent
    - Nested loops
    - Loop depth
    - Loop header, merge, continue and preheader
    - Basic blocks that belong to the loop

Correct a bug that forced dominator tree to be constantly rebuilt.