RFR: 8362284: RISC-V: cleanup NativeMovRegMem [v2]

[Hamlin Li]

> Hi,
> Can you help to review this simple patch?
> 
> NativeMovRegMem on riscv is actually dead code, but still needed in case of compilation of C1.
> So make the code as simple as possible to avoid any reading and maintainance effort.
> 
> No tests, as `offset()` and `set_offset()` were Unimplemented and used in C1 and never triggered before.
> 
> Thanks!

Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:

  use nullptr

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26328/files
  - new: https://git.openjdk.org/jdk/pull/26328/files/48c55caa..de5fa377

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26328&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26328&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26328.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26328/head:pull/26328

PR: https://git.openjdk.org/jdk/pull/26328