LLVM does not have an option same as GAS's `-malign-branch-prefix-size`. GAS' s ` -malign-branch-prefix-size` only elminates NOPs emitted by branch align, but clang's option `-mpad-max-prefix-size` aslo eliminates NOPs emitted by align directive. The two options are similar but slightly different.
My point is that we can make `-malign-branch-prefix-size` an alias for `-mpad-max-prefix-size` for user convenience when they need to prefix padding branches, but I am not sure whether it's appropriate. This revison is open mainly for discussion and I will not commit it if there is any objection.