[CodeGen] Rename functions PrintReg* to printReg*

LLVM Coding Standards:
  Function names should be verb phrases (as they represent actions), and
  command-like function should be imperative. The name should be camel
  case, and start with a lower case letter (e.g. openFile() or isFoo()).

Differential Revision: https://reviews.llvm.org/D40416

llvm-svn: 319168