arm: Adjust cost of vector of constant zero

arm: Adjust cost of vector of constant zero

Neon vector comparisons have a dedicated version when comparing with
constant zero: it means its cost is free.

Adjust the cost in arm_rtx_costs_internal accordingly, for Neon only,
since MVE does not support this.

2021-01-28  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	PR target/98730
	* config/arm/arm.c (arm_rtx_costs_internal): Adjust cost of vector
	of constant zero for comparisons.

	gcc/testsuite/
	PR target/98730
	* gcc.target/arm/simd/vceqzq_p64.c: Update expected result.