8361532: RISC-V: Several vector tests fail after JDK-8354383

Backport-of: e0245682c8d5a0daae055045c81248c12fb23c09

Author: DingliZhang

src/hotspot/cpu/riscv/riscv.ad

@@ -8431,6 +8431,17 @@ instruct castVV(vReg dst)
   ins_pipe(pipe_class_empty);
 %}
 
+instruct castVVMask(vRegMask dst)
+%{
+  match(Set dst (CastVV dst));
+
+  size(0);
+  format %{ "# castVV of $dst" %}
+  ins_encode(/* empty encoding */);
+  ins_cost(0);
+  ins_pipe(pipe_class_empty);
+%}
+
 // ============================================================================
 // Convert Instructions