RFR: 8332899: RISC-V: add comment and make the code more readable (if possible) in MacroAssembler::movptr

[Hamlin Li]

Hi,
Can you help to review the patch?
As discussed, https://github.com/openjdk/jdk/pull/19246#discussion_r1613279908, it's worth to make the code more readable.
For movptr1, add some comments to help understand the tricky part.
For movptr2, it uses the similar (tricky) way as movptr1, so I align the code implementation with movptr1, and try to make it more straightforward.
I tried it, hope it's better.
Thanks.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/19431/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19431&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332899
  Stats: 32 lines in 1 file changed: 25 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19431.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19431/head:pull/19431

PR: https://git.openjdk.org/jdk/pull/19431