RFR: 8292575: riscv: Represent Registers as values

Fri Aug 19 07:38:28 UTC 2022

Follow up [JDK-8292153](https://bugs.openjdk.org/browse/JDK-8292153) and [JDK-8292203](https://bugs.openjdk.org/browse/JDK-8292203), migrate away from pointer-based representation of Register values.

It improves compile-time checking by forbidding implicit conversions between integrals and pointers.

Testing:
- hotspot/jdk tier1 on unmatched board
- hotspot/jdk tier1 on qemu

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Commit messages:
 - Represent Registers as values

Changes: https://git.openjdk.org/jdk/pull/9936/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9936&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292575
  Stats: 509 lines in 14 files changed: 136 ins; 76 del; 297 mod
  Patch: https://git.openjdk.org/jdk/pull/9936.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9936/head:pull/9936

PR: https://git.openjdk.org/jdk/pull/9936