[CodeGen] Print constant pool index operands as %const.0 + 8 in both MIR and debug output

Work towards the unification of MIR and debug output by printing
`%const.0 + 8` instead of `<cp#0+8>` and `%const.0 - 8` instead of
`<cp#0-8>`.

Only debug syntax is affected.

Differential Revision: https://reviews.llvm.org/D41116

llvm-svn: 320564