[riscv-port] RFR: 8281731: riscv: Refactor instruction extraction code in nativeInst_riscv.h/cpp [v2]

[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 two additional commits since the last revision:

 - Remove local vars
 - A little optimization

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

Changes:
  - all: https://git.openjdk.java.net/riscv-port/pull/57/files
  - new: https://git.openjdk.java.net/riscv-port/pull/57/files/88a535d0..415a75c9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=57&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=57&range=00-01

  Stats: 100 lines in 2 files changed: 32 ins; 2 del; 66 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