RFR: 8308656: RISC-V: vstring_compare doesnt manifest usage of all vector registers [v2]

[Vladimir Kempik]

> Please review this fix. 
> vstring_compare instrinsic ( from c2_MacroAssembler_riscv.cpp ) uses vector registers v6 and v7 ( https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp#L1482 , vstr1 == v4, lmul=4) , but doesn't manifest their usage in riscv_v.ad file.
> This fix resolves this situation.
> No noticable difference one might see in generated code for now.
> 
> Testing: build testing only.

Vladimir Kempik has updated the pull request incrementally with one additional commit since the last revision:

  Revert part of the fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14102/files
  - new: https://git.openjdk.org/jdk/pull/14102/files/59612b8d..f385c66f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14102&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14102&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14102.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14102/head:pull/14102

PR: https://git.openjdk.org/jdk/pull/14102