[llvm-exegesis] Improve Register Setup.

Summary:
Added function to set a register to a particular value + tests.
Add EFLAGS test, use new setRegTo instead of setRegToConstant.

Reviewers: courbet, javed.absar

Subscribers: mgorny, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D51856

llvm-svn: 342466