[MIRVRegNamer] Experimental MachineInstr stable hashing (Fowler-Noll-Vo)

This hashing scheme has been useful out of tree, and I want to start
experimenting with it. Specifically I want to experiment on the
MIRVRegNamer, MIRCanononicalizer, and eventually the MachineOutliner.

This diff is a first step, that optionally brings stable hashing to the
MIRVRegNamer (and as a result, the MIRCanonicalizer).  We've tested this
hashing scheme on a lot of MachineOperand types that llvm::hash_value
can not handle in a stable manner.

This stable hashing was also the basis for

"Global Machine Outliner for ThinLTO" in EuroLLVM 2020

http://llvm.org/devmtg/2020-04/talks.html#TechTalk_58

Credits: Kyungwoo Lee, Nikolai Tillmann

Differential Revision: https://reviews.llvm.org/D86952