Add loop recognition and CFG simplifications in new compiler.

We do three simplifications:
- Split critical edges, for code generation from SSA (new).
- Ensure one back edge per loop, to simplify loop recognition (new).
- Ensure only one pre header for a loop, to simplify SSA creation (existing).

Change-Id: I9bfccd4b236a00486a261078627b091c8a68be33