[GVN] Preserve loop related analysis/canonical forms.

LoopInfo can be easily preserved by passing it to the functions that
modify the CFG (SplitCriticalEdge and MergeBlockIntoPredecessor.
SplitCriticalEdge also preserves LoopSimplify and LCSSA form when when passing in
LoopInfo. The test case shows that we preserve LoopSimplify and
LoopInfo. Adding addPreservedID(LCSSAID) did not preserve LCSSA for some
reason.

Also I am not sure if it is possible to preserve those in the new pass
manager, as they aren't analysis passes.

Reviewers: reames, hfinkel, davide, jdoerfert

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D65137

llvm-svn: 367332