[PATCH][AArch64] Handle aliases of conditional branches without b.pred form.

[Tim Northover]

> Why? Are we going to start accepting "rN" as an alias for "wN" next?
> AArch64 is not AArch32, and anyone writing code for the former should
> be using "b.pred".

To clarify, I think the right solution to this "problem" is getting
binutils to remove support for this incorrect syntax. If they can't do
that immediately they should deprecate it (with a warning) and remove
it later.

I'd reluctantly go along with us adding it temporarily for
compatibility with such a warning. But this needs to be discussed with
GNU folks first.

Cheers.

Tim.