Implement if-conversion.

Requires Cost-Model JDK-8340093, and some way to modify the scalar graph: convert CFG to predicated/CMove/Masked code.

Element-wise loops without additional exits should be easy.

More complicated patterns:
- conditional exits in each iteration, such as search loop

These will require some sort of "reduction/prefix-and" over the conditions, and then disable the lanes that are after the exit.

This may take a lot of experimenting, and breaking up into different subtasks.