cmd/internal/obj/arm64: fix the size of STP series instructions in optab
cmd/internal/obj/arm64: fix the size of STP series instructions in optab

When the class of p.To is C_NAUTO4K, STP series instructions will be translated
into add/sub + stp instructions, the total size is 8. Currently this size
value in optab is 12, this CL fixes it.

Change-Id: Ife1b9e9f8a28edca9b59f62b45a80d83f38993b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/314210
Reviewed-by: eric fang <68c7b5ebe4a86ef7e52157f5950041656bf82310@arm.com>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Trust: eric fang <68c7b5ebe4a86ef7e52157f5950041656bf82310@arm.com>
Run-TryBot: eric fang <68c7b5ebe4a86ef7e52157f5950041656bf82310@arm.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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