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

[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.

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

Commit messages:
 - 8308656: RISC-V: vstring_compare doesnt manifest usage of all vector registers

Changes: https://git.openjdk.org/jdk/pull/14102/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14102&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308656
  Stats: 42 lines in 2 files changed: 34 ins; 0 del; 8 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