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.