[AArch64] Keep flags on condition vreg when instantiating a CB branch.

Reversing a CB* instruction used to drop the flags on the condition. On the
included testcase, this lead to a read from an undefined vreg.
Using addOperand keeps the flags, here <undef>.

Differential Revision: http://reviews.llvm.org/D6159


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221507 91177308-0d34-0410-b5e6-96231b3b80d8