Use OpReturn* in wrap-opkill (#2886)

* Use OpReturn* in wrap-opkill

The warp-opkill pass is generating incorrect code.  It is placing an
OpUnreachable at the end of a basic block, when the block can be
reached.  We can't reach the end of the block, but we can reach the end.
Instead we will add a return instruction.

Fixes #2875.