[esan|cfrag] Compute the struct field access difference ratio

Summary:
Computes the struct field access variation based on each field access
count.

Adds a flag to control the report thresholds.

Updates struct-simple.cpp with variance report output.

Reviewers: aizatsky

Subscribers: kubabrecka, zhaoqin, llvm-commits, eugenis, vitalybuka, kcc, bruening

Differential Revision: http://reviews.llvm.org/D20914

llvm-svn: 271734