RFR: heapbase register can be allocated in compressed mode

[Kuai Wei]

Hi,

  I made an enhancement for aarch64 platform. It's based on great work of https://bugs.openjdk.java.net/browse/JDK-8233743
and .

  In compressed oops mode , if heapbase is zero, jvm don't use heapbase register to encode/decode. So it can be allocated by
JIT compiler.

The webrev is:
http://cr.openjdk.java.net/~wzhuo/8242449/webrev.00/

The bug link:
https://bugs.openjdk.java.net/browse/JDK-8242449

Thanks,
Kuai Wei