Fixes #2338. Added functionality to remove OpPhi instructions (replacing...

Fixes #2338.  Added functionality to remove OpPhi instructions (replacing their uses) when merging blocks (#2339)

* Fixes #2338.  Added check for phi node before merging blocks.

* Added functionality to merge blocks A and B even when B starts with OpPhi instructions, by replacing uses of the OpPhi results with the definitions coming from A.  Added some tests for this.

* Fixed assertion.