[CodeGen] Print stack object references as %(fixed-)stack.0 in both MIR and debug output

Work towards the unification of MIR and debug output by printing
`%stack.0` instead of `<fi#0>`, and `%fixed-stack.0` instead of
`<fi#-4>` (supposing there are 4 fixed stack objects).

Only debug syntax is affected.

Differential Revision: https://reviews.llvm.org/D41027

llvm-svn: 320827