[CodeGen] Use MachineInstr::getOperand(0) instead of gets the defs iterator_range and calling begin. NFC

Defs are well defined to come first in MachineInstr operand list. No need for a more complex indirection.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332559 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed