Remove the LoopInstSimplify pass (-loop-instsimplify)

LoopInstSimplify is unused and untested. Reading through the commit
history the pass also seems to have a high maintenance burden.

It would be best to retire the pass for now. It should be easy to
recover if we need something similar in the future.

Differential Revision: https://reviews.llvm.org/D44053

llvm-svn: 327329