RFR: 8135231: add support for vectorizing double precision sqrt

[Roland Westrelin]

> http://cr.openjdk.java.net/~enevill/8135231/webrev/

That looks good to me but someone who actually knows AARCH64 needs to take a look.

> There is a trivial change to test/compiler/loopopts/superword/SumRedSqrt_Double.java
> 
> -* @requires os.arch=="x86" | os.arch=="amd64" | os.arch=="x86_64"
> +* @requires os.arch=="x86" | os.arch=="amd64" | os.arch=="x86_64" | os.arch=="aarch64"
> 
> I am not clear whether this change means that the change requires sponsoring and pushing through JPRT, but if so could someone do this.

I don’t think we make any exception so I’ll push it once you get another review.

Roland.