AArch64: Improve strlen_asimd
Use shrn for the mask, merge tst+bne into cbnz, and tweak code alignment.
Performance improves slightly as a result.
Reviewed-by: 
Szabolcs Nagy <szabolcs.nagy@arm.com>
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Use shrn for the mask, merge tst+bne into cbnz, and tweak code alignment.
Performance improves slightly as a result.
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>