[Cloner] Re-map simplfied cloned instructions.

This commit pretty much rolls back the logic added in r306495
as in the testcase provided we simplify an `icmp` looking through
a PHI that hasn't been mapped yet.

I think instsimplify shouldn't do threading over select/phis or
just looking through phis in general, but this is what we have
now. Also, add a test to prevent this from happening in case somebody
wants to modify this code again.

Briefly discussed with Kyle Butt (thanks Kyle!).

llvm-svn: 306938