[NFC][llvm-exegesis] Refactor Analysis::SchedClassCluster::measurementsMatch()

Summary:
The diff looks scary but it really isn't:
1. I moved the check for the number of measurements into `SchedClassClusterCentroid::validate()`
2. While there, added a check that we can only have a single inverse throughput measurement. I missed that when adding it initially.
3. In `Analysis::SchedClassCluster::measurementsMatch()` is called with the current LLVM values from schedule class and the values from Centroid.
3.1. The values from centroid we can already get from `SchedClassClusterCentroid::getAsPoint()`.
     This isn't 100% a NFC, because previously for inverse throughput we used `min()`. I have asked whether i have done that correctly in
     https://reviews.llvm.org/D57647?id=184939#inline-510384 but did not hear back. I think `avg()` should be used too, thus it is a fix.
3.2. Finally, refactor the computation of the LLVM-specified values into `Analysis::SchedClassCluster::getSchedClassPoint()`
     I will need that function for [[ https://bugs.llvm.org/show_bug.cgi?id=41275 | PR41275 ]]

Reviewers: courbet, gchatelet

Reviewed By: courbet

Subscribers: tschuett, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59951

llvm-svn: 357245