Rewriting dead branch elimination.

Rewriting dead branch elimination.

Pass now paints live blocks and fixes constant branches and switches as
it goes. No longer requires structured control flow. It also removes
unreachable blocks as a side effect. It fixes the IR (phis) before doing
any code removal (other than terminator changes).

Added several unit tests for updated/new functionality.

Does not remove dead edge from a phi node:
* Checks that incoming edges are live in order to retain them
* Added BasicBlock::IsSuccessor
* added test

Fixing phi updates in the presence of extra backedge blocks

* Added tests to catch bug

Reworked how phis are updated

* Instead of creating a new Phi and RAUW'ing the old phi with it, I now
replace the phi operands, but maintain the def/use manager correctly.

For unreachable merge:

* When considering unreachable continue blocks the code now properly
checks whether the incoming edge will continue to be live.

Major refactoring for review

* Broke into 4 major functions
 * marking live blocks
 * marking structured targets
 * fixing phis
 * deleting blocks