RISC-V: Fix one warning of maybe-uninitialized in riscv-vsetvl.cc

RISC-V: Fix one warning of maybe-uninitialized in riscv-vsetvl.cc

This patch would like to fix one maybe-uninitialized warning. Aka:

riscv-vsetvl.cc:4354:3: error: 'vsetvl_rinsn' may be used uninitialized [-Werror=maybe-uninitialized]

Signed-off-by: Pan Li <pan2.li@intel.com>
gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc
	(pass_vsetvl::global_eliminate_vsetvl_insn): Initialize var by NULL.