Do not inline OpKill Instructions (#2713)

It is illegal to inline an OpKill instruction into a continue construct because the continue header will no longer dominate the backedge.

This commit adds a check for this, and does not inline.

If we still want to be able to inline a function that contains an OpKill, we can add a new pass that will wrap OpKill instructions into its own function with just the single instruction.

I do not believe that this is a common case right now, so I will not do that yet.

Fixes #2433.