[NFCi][llvm][MIRVRegNamerUtils] Making some code cleanup and stylistic changes.

Making some changes to MIRVRegNamerUtils.cpp to use some more modern c++
features as well as some changes to generally make the code more concise
and more understandable.

I make this an NFCi because in one case I drop the whole
"if (!MO->isDef()) MO->setIsKill(false);" thing that was added in the
original implementation, generally because I don't think this is really
semantically sound. I also changed up the implementation of
VRegRenamer::createVirtualRegisterWithLowerName somewhat because I am
now lower-casing the name unconditionally because I confirmed that that
was in fact aditya_nandakumar@apple.com's intent.

In all other cases, behavior should not be changed.

Differential Revision: https://reviews.llvm.org/D71182