RFR: 8308091: Remove unused iRegIHeapbase() matching operand

[Xiaolin Zheng]

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

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Commit messages:
 - Remove unused iRegIHeapbase() matching operand

Changes: https://git.openjdk.org/jdk/pull/13983/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13983&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308091
  Stats: 21 lines in 2 files changed: 0 ins; 21 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13983.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13983/head:pull/13983

PR: https://git.openjdk.org/jdk/pull/13983