[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.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215431 91177308-0d34-0410-b5e6-96231b3b80d8
Have MachineRegisterInfo take and store the MachineFunction it
was created for rather than the TargetMachine since we only
needed the TM for the subtarget and we can get that from the
MF.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215432 91177308-0d34-0410-b5e6-96231b3b80d8