cmd/compile: store the comparison pseudo-ops of arm64 conditional instructions in...

cmd/compile: store the comparison pseudo-ops of arm64 conditional instructions in AuxInt

The current implementation stores the comparison pseudo-ops of arm64
conditional instructions (CSEL/CSEL0) in Aux, this patch modifies it
and stores it in AuxInt, which can avoid the allocation.

Change-Id: I0b69e51f63acd84c6878c6a59ccf6417501a8cfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/252517
Run-TryBot: fannie zhang <Fannie.Zhang@arm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>