[XLA] Fix HLO graph dumper not to assume that instruction names start with "%".

HLO graph dumper needs to be aware that we've gotten rid of the "%"
prefix in HLO names so it doesn't print e.g.

  reduce
  reduce.42
  Subcomputation: add
  ...

but instead simply prints

  reduce.42
  Subcomputation: add
  ...

PiperOrigin-RevId: 179756922