[AArch64] PR79262: Adjust vector cost

[AArch64] PR79262: Adjust vector cost

PR79262 has been fixed for almost all AArch64 cpus, however the example is still
vectorized in a few cases, resulting in lower performance.  Adjust the vector
cost slightly so that so that -mcpu=cortex-a53 now has identical performance as
-mcpu=cortex-a57 on libquantum.

    gcc/
	PR target/79262
	* config/aarch64/aarch64.c (generic_vector_cost): Adjust
	vec_to_scalar_cost.

From-SVN: r278452