[CodeGen] Print register names in lowercase in both MIR and debug output

As part of the unification of the debug format and the MIR format,
always print registers as lowercase.

* Only debug printing is affected. It now follows MIR.

Differential Revision: https://reviews.llvm.org/D40417

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319187 91177308-0d34-0410-b5e6-96231b3b80d8
358 files changed