RFR: 8293566: RISC-V: Clean up push and pop registers [v4]

[Feilong Jiang]

> Currently, riscv port has two implementations of `push_reg` and `pop_reg`.
> The bitset version can be replaced with the RegSet version for better readability.
> 
> For example, if we want to push x10 and x11, pushing register with bitset would be like:
> 
> push_reg(0xc00, sp)
> 
> 
> while pushing register with RegSet will be more clear:
> 
> push_reg(RegSet::of(x10, x11), sp);
> 
> 
> Testing:
> 
> - hotspot and jdk ter1 on QEMU without new failures

Feilong Jiang has updated the pull request incrementally with one additional commit since the last revision:

  add comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10227/files
  - new: https://git.openjdk.org/jdk/pull/10227/files/5dd1c9ef..82765710

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10227&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10227&range=02-03

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10227.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10227/head:pull/10227

PR: https://git.openjdk.org/jdk/pull/10227