[LoopRotate] Add test case to show dbg value problem

Summary:
In commit d60f34c2 (llvm-svn 317128,
PR35113) MergeBlockIntoPredecessor was changed into
discarding some dbg.value intrinsics referring to
PHI values, post-splice due to loop rotation.

That elimination of dbg.value intrinsics does not
consider which dbg.value to keep based on the context.
Such as always keeping the one that comes first textually,
or the need to keep several of them in case the variable
is changing it's value several times inside the basic block.

In the past that hasn't been such a big problem since
CodeGenPrepare::placeDbgValues has moved the dbg.value
to be next to the PHI node anyway. But after commit
00e23889 CodeGenPrepare isn't doing that
any longer, so we need to be more careful when avoiding
duplicate dbg.value intrinsics in MergeBlockIntoPredecessor.

This patch is just a pre commit of the test case.

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71479