Reland "[CodeGen] Fix unconditional branch duplication issue in bbsections"

Reverted in 4c8d056f because it broke
buildbot `llvm-clang-x86_64-expensive-checks-debian` due to the AArch64
test generating invalid code. The issue still exists, but it's fixed in
D156767, so the AArch64 test should be added there.

Differential Revision: https://reviews.llvm.org/D158674