When assigning a physical register to a MachineOperand, set

When assigning a physical register to a MachineOperand, set
the subreg field to 0, since the subreg field is only used
for virtual register subregs. This doesn't change
current functionality; it just eliminates bogus noise from
debug output.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68955 91177308-0d34-0410-b5e6-96231b3b80d8