RISC-V: Make full-vec-move1.c test robust for optimization

RISC-V: Make full-vec-move1.c test robust for optimization

During investigate the support of early break autovec, we notice
the test full-vec-move1.c will be optimized to 'return 0;' in main
function body.  Because somehow the value of V type is compiler
time constant,  and then the second loop will be considered as
assert (true).

Thus,  the ccp4 pass will eliminate these stmt and just return 0.

typedef int16_t V __attribute__((vector_size (128)));

int main ()
{
  V v;
  for (int i = 0; i < sizeof (v) / sizeof (v[0]); i++)
    (v)[i] = i;

  V res = v;
  for (int i = 0; i < sizeof (v) / sizeof (v[0]); i++)
    assert (res[i] == i); // will be optimized to assert (true)
}

This patch would like to introduce a extern function to use the res[i]
that get rid of the ccp4 optimization.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls-vlmax/full-vec-move1.c:
	Introduce extern func use to get rid of ccp4 optimization.

Signed-off-by: Pan Li <pan2.li@intel.com>
(cherry picked from commit b1520d2260c5e0cfcd7a4354fab70f66e2912ff2)