Set the parent pointer of cloned DBG_VALUE instructions correctly.
Fixes PR20523.

When spilling variables onto the stack, spillVirtReg() is setting the
parent pointer of the cloned DBG_VALUE intrinsic for the stack location
to the parent pointer of the original intrinsic. MachineInstr parent
pointers should however always point to the parent basic block.

MBB is shadowing the MBB member variable. The instruction still ends up
being inserted into the right basic block, because it's inserted after MI
which serves as the iterator.

I failed at constructing a reliable testcase for this, see
http://llvm.org/bugs/show_bug.cgi?id=20523 for a large testcases.