RFR: 8345110: RISC-V: Optimize and and clean up byte reverse assembler routines [v2]

[Fei Yang]

> Hi, Could you please help review this change?
> 
> MacroAssembler routines `revb_w` and `revb_w_w` will emit 32 and 17 instructions respectively without Zbb.
> 
> Major changes:
> 1. There is no need for `revb_w` to reverse bytes for each 32-bit word in the 64-bit register.
>    All the callers only care about the lower 32-bit word. This reimplement `revb_w` so that it only
>    reverses bytes in lower 32-bit word and sign-extends the result (14 instructions). This makes `revb_w_w` duplicate.
> 2. Removed some assembler routines that are unused or become unnecessary (`revb_h`, `revb_h_helper`, `revb_h_h`, `revb_w_w`).
> 3. Adapted callers of these removed assembler routines in file `riscv_b.ad` to emit Zbb instructions directly.
> 
> Will look into `revb_h_h_u` and `revb_h_w_u` after this change. Seems to me they could be factored out as well.
> 
> Testing on linux-riscv64 platform:
> - [x] tier1 (release)
> - [x] hotspot:tier1 (fastdebug)
> - [x] non-trivial benchmark workloads like Dacapo, SpecJBB, Renaissance (release)

Fei Yang has updated the pull request incrementally with one additional commit since the last revision:

  Review comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22410/files
  - new: https://git.openjdk.org/jdk/pull/22410/files/f737241a..7436021c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22410&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22410&range=00-01

  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22410.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22410/head:pull/22410

PR: https://git.openjdk.org/jdk/pull/22410