[esan|cfrag] Handle complex GEP instr in the cfrag tool

Summary:
Iterates all (except the first and the last) operands within each GEP
instruction for instrumentation.

Adds test struct_field_gep.ll.

Reviewers: aizatsky

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

Differential Revision: http://reviews.llvm.org/D21242

llvm-svn: 272442