Updated llvm-proto-fuzzer to execute the compiled code

Summary:
Made changes to the llvm-proto-fuzzer
- Added loop vectorizer optimization pass in order to have two IR versions
- Updated old fuzz target to handle two different IR versions
- Wrote code to execute both versions in memory

Reviewers: morehouse, kcc, alexshap

Reviewed By: morehouse

Subscribers: pcc, mgorny, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D49526

llvm-svn: 338077