[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.

llvm-svn: 332559

llvm/lib/CodeGen/ImplicitNullChecks.cpp

@@ -596,7 +596,7 @@ MachineInstr *ImplicitNullChecks::insertFaultingInstr(
 
   unsigned DefReg = NoRegister;
   if (NumDefs != 0) {
-    DefReg = MI->defs().begin()->getReg();
+    DefReg = MI->getOperand(0).getReg();
     assert(NumDefs == 1 && "expected exactly one def!");
   }