RISC-V: Fix vl_used_by_non_rvv_insn logic of vsetvl pass

RISC-V: Fix vl_used_by_non_rvv_insn logic of vsetvl pass

This patch fixes a bug in the current vsetvl pass.  The current pass uses
`m_vl` to determine whether the dest operand has been used by non-RVV
instructions.  However, `m_vl` may have been modified as a result of an
`update_avl` call, and thus would be no longer the dest operand of the
original instruction.  This can lead to incorrect vsetvl eliminations, as is
shown in the testcase.  In this patch, we create a `dest_vl` variable for
this scenerio.

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc: Use `dest_vl` for dest VL operand

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/vsetvl/vsetvl_bug-3.c: New test.