[aarch64-port-dev ] RFR: 8191955: AArch64: incorrect prefetch distance causes an internal error

[Ningsheng Jian]

Hi,

Please help to review this small fix for AArch64.

JBS:
https://bugs.openjdk.java.net/browse/JDK-8191955
(with some descriptions)

Webrev:
http://cr.openjdk.java.net/~njian/8191955/webrev.00/

An unexpected distance triggers the error for PRFM instruction
generation. This patch will warn and fix the unexpected input value.
jtreg passed with fastdebug build.

OK for 10?

Thanks,
Ningsheng