[mir-canon][NFC] Move MIR vreg renaming code to separate file for better reuse.

Moving MIRCanonicalizerPass vreg renaming code to MIRVRegNamerUtils so that it
can be reused in another pass (ie planing to write a standalone mir-namer pass).

I'm going to write a mir-namer pass so that next time someone has to author a
test in MIR, they can use it to cleanup the naming and make it more readable by
having the numbered vregs swapped out with named vregs.

Differential Revision: https://reviews.llvm.org/D67114

llvm-svn: 370985