SelectionDAG, MI, AArch64: Widen target flags fields/arguments from unsigned char to unsigned.

This makes the field wider than MachineOperand::SubReg_TargetFlags so that
we don't end up silently truncating any higher bits. We should still catch
any bits truncated from the MachineOperand field as a consequence of the
assertion in MachineOperand::setTargetFlags().

Differential Revision: https://reviews.llvm.org/D65465

llvm-svn: 367474