[esan|cfrag] Instrument GEP instr for struct field access.

Summary:
Instrument GEP instruction for counting the number of struct field
address calculation to approximate the number of struct field accesses.

Adds test struct_field_count_basic.ll to test the struct field
instrumentation.

Reviewers: bruening, aizatsky

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

Differential Revision: http://reviews.llvm.org/D20892

llvm-svn: 271619