8296771: RISC-V: C2: assert(false) failed: bad AD file

Reviewed-by: shade, fyang

Author: zhengxiaolinX

src/hotspot/cpu/riscv/riscv.ad

@@ -3265,6 +3265,7 @@ operand iRegP()
   match(RegP);
   match(iRegPNoSp);
   match(iRegP_R10);
+  match(iRegP_R15);
   match(javaThread_RegP);
   op_cost(0);
   format %{ %}