[MachineTraceMetrics] Add computeDepth function (NFCI).

Summary:
This function is used in D36619 to update the instruction depths
incrementally.

Reviewers: efriedma, Gerolf, MatzeB, fhahn

Reviewed By: fhahn

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36696

llvm-svn: 312714