The `iRegIHeapbase()` matching operand has no usage on both AArch64 and RISC-V platforms after JDK-8242449 and JDK-8306667, respectively. As the following-up action discussed in the code review process of JDK-8306667, this is a small cleanup for the `iRegIHeapbase()` matching operand.

For AArch64, the `Affects Version` should be 15~21; for RISC-V, the `Affects Version` is 21.