[PATCH] RISC-V: Fix SEW64 of vrsub.vx runtime fail in RV32 system

It's quite obvious that the order of vrsub SEW64 is wrong.

gcc/ChangeLog:

	* config/riscv/vector.md: Fix incorrect operand order.

gcc/testsuite/ChangeLog:

	* g++.target/riscv/rvv/base/bug-23.C: New test.