[MachineCombiner] Fix for ICE bug 20598

The combiner ignored DBG nodes when checking
the uses of a virtual register.

It combined a sequence like
   %vreg1 = madd %vreg2, %vreg3,...
   DBG_VALUE (%vreg1 ...)
   %vreg4 = add %vreg1,...
to
  %vreg4 = madd %vreg2, %vreg3

leaving behind a dangling DBG_VALUE with
a definition. This triggered an assertion
in the MachineTraceMetrics.cpp module.