[CodeGen] Always use `printReg` to print registers in both MIR and debug
output

As part of the unification of the debug format and the MIR format,
always use `printReg` to print all kinds of registers.

Updated the tests using '_' instead of '%noreg' until we decide which
one we want to be the default one.

Differential Revision: https://reviews.llvm.org/D40421

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319445 91177308-0d34-0410-b5e6-96231b3b80d8
138 files changed