[LoopFusion] Extend use of OptimizationRemarkEmitter

Summary:
This patch extends the use of the OptimizationRemarkEmitter to provide
information about loops that are not fused, and loops that are not eligible for
fusion. In particular, it uses the OptimizationRemarkAnalysis to identify loops
that are not eligible for fusion and the OptimizationRemarkMissed to identify
loops that cannot be fused.

It also reuses the statistics to provide the messages used in the
OptimizationRemarks. This provides common message strings between the
optimization remarks and the statistics.

I would like feedback on this approach, in general. If people are OK with this,
I will flesh out additional remarks in subsequent commits.

Subscribers: hiraditya, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63844

llvm-svn: 367327