Use <def,undef> operands when spilling NEON bundles.

MachineOperands that define part of a virtual register must have an
<undef> flag if they are not intended as read-modify-write operands.

The old trick of adding an <imp-def> operand doesn't work any longer.

Fixes PR12177.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152008 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineInstrBuilder.h

@@ -34,6 +34,7 @@ namespace RegState {
     Undef          = 0x20,
     EarlyClobber   = 0x40,
     Debug          = 0x80,
+    DefineNoRead   = Define | Undef,
     ImplicitDefine = Implicit | Define,
     ImplicitKill   = Implicit | Kill
   };