[MIR] MIRNamer pass for improving MIR test authoring experience.

This patch reuses the MIR vreg renamer from the MIRCanonicalizerPass to cleanup
names of vregs in a MIR file for MIR test authors. I found it useful when
writing a regression test for a globalisel failure I encountered recently and
thought it might be useful for other folks as well.

Differential Revision: https://reviews.llvm.org/D67209

llvm-svn: 371121