[ARM/AARCH64] Make test MachineBranchProb.ll more robust and re-enable for ARM/AArch64

Summary: Make test robust enough to not fail due to CFG changes and re-enable for ARM/AArch64.

Reviewers: rovka, fhahn

Reviewed By: fhahn

Subscribers: fhahn, aemerson, rengolin, mcrosier, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D38590

llvm-svn: 315002