RFR: 8292575: riscv: Represent Registers as values [v4]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' of https://github.com/openjdk/jdk into riscv-register-rep
 - remove duplicate code
 - Merge branch 'master' of https://github.com/openjdk/jdk into riscv-register-rep
 - Represent Registers as values

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9936/files
  - new: https://git.openjdk.org/jdk/pull/9936/files/b9dbd6b4..ba493543

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9936&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9936&range=02-03

  Stats: 1623 lines in 101 files changed: 1195 ins; 150 del; 278 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