RFR: 8292575: riscv: Represent Registers as values [v3]

[Feilong Jiang]

> 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

Feilong Jiang has updated the pull request incrementally with one additional commit since the last revision:

  remove duplicate code

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9936/files
  - new: https://git.openjdk.org/jdk/pull/9936/files/72e55d46..b9dbd6b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9936&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9936&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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