[NVPTX] Copy machine operand flags in TII::insertBranch

Before this patch, flags such as undef were dropped by TII::insertBranch
(used by BranchFolding pass), resulting in the following error from
machine verifier:

    *** Bad machine code: Reading virtual register without a def ***
    - function:    hoge
    - basic block: %bb.0 bb (0x562e9c240e68)
    - instruction: CBranch %2:int1regs, %bb.3
    - operand 0:   %2:int1regs

Differential Revision: https://reviews.llvm.org/D113001