RISC-V: Fix ICE in extract_single_source

This patch fixes the following ICE in VSETVL PASS:
bug.c:39:1: internal compiler error: Segmentation fault
   39 | }
      | ^
0x1ad5a08 crash_signal
        ../../../../gcc/gcc/toplev.cc:316
0x7f7f55feb90f ???
        ./signal/../sysdeps/unix/sysv/linux/x86_64/libc_sigaction.c:0
0x218d7c7 extract_single_source
        ../../../../gcc/gcc/config/riscv/riscv-vsetvl.cc:583
0x218d95d extract_single_source
        ../../../../gcc/gcc/config/riscv/riscv-vsetvl.cc:604
0x218fbc5 pre_vsetvl::compute_lcm_local_properties()
        ../../../../gcc/gcc/config/riscv/riscv-vsetvl.cc:2703
0x2190ef4 pre_vsetvl::earliest_fuse_vsetvl_info()
        ../../../../gcc/gcc/config/riscv/riscv-vsetvl.cc:2890
0x2193e62 pass_vsetvl::lazy_vsetvl()
        ../../../../gcc/gcc/config/riscv/riscv-vsetvl.cc:3537
0x219406a pass_vsetvl::execute(function*)
        ../../../../gcc/gcc/config/riscv/riscv-vsetvl.cc:3584

The rootcause we have a case that the def info can not be traced:

(insn 208 327 333 27 (use (reg/i:DI 10 a0)) "bug.c":36:1 -1
     (nil))

It's obvious, we conservatively disable any optimization in this situation if
AVL def_info can not be tracded.

Committed as it is obvious.

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc (extract_single_source): Fix ICE.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/vsetvl/avl_use_bug-1.c: New test.