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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371121 91177308-0d34-0410-b5e6-96231b3b80d8