[ARM] Try to re-enable MachineBranchProb.ll for ARM/AArch64

Commit r298799 changed code that made the XFAIL on MachineBranchProb.ll
irrelevant, but some configurations still failed. I can't reproduce it
locally, so I'm hoping that enabling this will tell me if some
configurations will really fail or if they were just too slow.

llvm-svn: 299558