Restore the LoopInstSimplify pass, reverting r327329 that removed it.

The plan had always been to move towards using this rather than so much
in-pass simplification within the loop pipeline, but we never got around
to it.... until only a couple months after it was removed due to disuse.
=/

This commit is just a pure revert of the removal. I will add tests and
do some basic cleanup in follow-up commits. Then I'll wire it into the
loop pass pipeline.

Differential Revision: https://reviews.llvm.org/D47353

llvm-svn: 333250
diff --git a/llvm/lib/Transforms/Scalar/Scalar.cpp b/llvm/lib/Transforms/Scalar/Scalar.cpp
index 36f5ba7..75fd921 100644
--- a/llvm/lib/Transforms/Scalar/Scalar.cpp
+++ b/llvm/lib/Transforms/Scalar/Scalar.cpp
@@ -62,6 +62,7 @@
   initializeLoopDataPrefetchLegacyPassPass(Registry);
   initializeLoopDeletionLegacyPassPass(Registry);
   initializeLoopAccessLegacyAnalysisPass(Registry);
+  initializeLoopInstSimplifyLegacyPassPass(Registry);
   initializeLoopInterchangePass(Registry);
   initializeLoopPredicationLegacyPassPass(Registry);
   initializeLoopRotateLegacyPassPass(Registry);