[llvm-exegesis] Analysis: detect clustering inconsistencies.

Summary:
Warn on instructions that should have the same performance
characteristics according to the sched model but actually
differ in their benchmarks.

Next step: Make the display nicer to browse, I was thinking maybe html.

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D46945

llvm-svn: 332601