RFR: 8308091: Remove unused iRegIHeapbase() matching operand

[Fei Yang]

On Mon, 15 May 2023 10:56:26 GMT, Xiaolin Zheng <xlinzheng at openjdk.org> wrote:

> The `iRegIHeapbase()` matching operand has no usage on both AArch64 and RISC-V platforms after [JDK-8242449](https://bugs.openjdk.org/browse/JDK-8242449) and [JDK-8306667](https://bugs.openjdk.org/browse/JDK-8306667), respectively. As the following-up action discussed in the code review process of [JDK-8306667](https://bugs.openjdk.org/browse/JDK-8306667) (#13577), this is a small cleanup for the `iRegIHeapbase()` matching operand.
> 
> Passed fastdebug/release build on both AArch64/RISC-V platforms.
> 
> Thanks,
> Xiaolin

Maybe we should remove `reg_class heapbase_reg` at the same time?

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PR Review: https://git.openjdk.org/jdk/pull/13983#pullrequestreview-1427572557