[MIPS] BuildCondBr should preserve MO flags

While simplifying branches in the MachineInstr representation, the
routine BuildCondBr must preserve flags on register MachineOperands. In
particular, it must preserve the <undef> flag.

This fixes a bug that is unlikely to occur in any real scenario, but
which bugpoint is likely to introduce.

Patch By Nick Johnson!

Reviewers: ahatanak, sdardis

Differential Revision: https://reviews.llvm.org/D34041

llvm-svn: 305290