RFR: 8332533: RISC-V: Enable vector variable shift instructions for machines with RVV

[Gui Cao]

Hi, I noticed the following warning in the Opto JIT Code for the Vector API in the `test/jdk/jdk/incubator/vector/Byte256VectorTests.java: ASHRByte256VectorTests` test:

--------------------------------------------------------------------------------
  ** Rejected vector op (RShiftVB,byte,32) because architecture does not support variable vector shifts
  ** not supported: arity=2 opc=405 vlen=32 etype=byte ismask=0 is_masked_op=0
``` 
the reason is because Matcher::supports_vector_ variable_shifts returns false. the port of RISC-V Vector API now supports the vector shifts, so this should return with UseRVV.  By the Way, the Matcher::supports_vector_variable_shifts function was introduced by Vector API, and I think forgot to modify the Matcher::supports_vector_variable_shifts function when implementing vector shift.
After the fix, the test passes normally and generates the Opto JIT Code such as:

1c2     loadV V1, [R7]  # vector (rvv)
1ca     lwu  R28, [R28, #12]    # loadN, compressed ptr, #@loadN ! Field: jdk/internal/vm/vector/VectorSupport$VectorPayload.payload (constant)
1ce     decode_heap_oop  R7, R28    #@decodeHeapOop
1d2     addi  R7, R7, #16   # ptr, #@addP_reg_imm
1d4     loadV V2, [R7]  # vector (rvv)
1dc     vand_immI V1, V1, #7
1e4     spill [sp, #48] -> R7    # spill size = 32
1e6     # castII of R7, #@castII
1e6     vasrB V3, V2, V1
1fa     spill [sp, #96] -> R29   # spill size = 32
1fc     bgeu  R7, R29, B101 #@cmpU_branch  P=0.000001 C=-1.000000


### Testing:
qemu 8.1.50 with UseRVV:
- [ ] Run tier1-3 tests (release)
- [x] Run test/jdk/jdk/incubator/vector (fastdebug)

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

Commit messages:
 - 8332533: RISC-V: Enable Matcher::supports_vector_variable_shifts with UseRVV

Changes: https://git.openjdk.org/jdk/pull/19313/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19313&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332533
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19313.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19313/head:pull/19313

PR: https://git.openjdk.org/jdk/pull/19313