[riscv-port] RFR: 8281731: riscv: Refactor instruction extraction code in nativeInst_riscv.h/cpp [v3]

[Xiaolin Zheng]

> Hi team,
> 
> Could I have a review of this patch - this is just a trivial refactoring with some cleanups for `nativeInst_riscv.h/cpp` using `Assembler::extract()` with raw bits, by adding `extract_rs1`, `extract_rs2`, `extract_rd`, `extract_opcode`, and `extract_funct3` to make the code cleaner and conciser. Tested hotspot tier1 on Qemu with no other errors found.
> 
> Thanks,
> Xiaolin

Xiaolin Zheng has updated the pull request incrementally with one additional commit since the last revision:

  Keep readability

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

Changes:
  - all: https://git.openjdk.java.net/riscv-port/pull/57/files
  - new: https://git.openjdk.java.net/riscv-port/pull/57/files/415a75c9..a48557b6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=57&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=57&range=01-02

  Stats: 80 lines in 1 file changed: 0 ins; 31 del; 49 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/57.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/57/head:pull/57

PR: https://git.openjdk.java.net/riscv-port/pull/57