[JumpThreading] Restrict PRE across instructions that don't pass control to successors

PRE in JumpThreading should not be able to hoist copy of non-speculable loads across
instructions that don't always transfer execution to their successors, otherwise they may
introduce an unsafe load which otherwise would not be executed.

The same problem for GVN was fixed as rL316975.

Differential Revision: https://reviews.llvm.org/D40347

llvm-svn: 321063