cmd/compile: optimize codes with arm64 REV16 instruction

Optimize some patterns into rev16/rev16w instruction.

Pattern1:
    (c & 0xff00ff00)>>8 | (c & 0x00ff00ff)<<8
To:
    rev16w c

Pattern2:
    (c & 0xff00ff00ff00ff00)>>8 | (c & 0x00ff00ff00ff00ff)<<8
To:
    rev16 c

This patch is a copy of CL 239637, contributed by Alice Xu(dianhong.xu@arm.com).

Change-Id: I96936c1db87618bc1903c04221c7e9b2779455b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/268377


Trust: fannie zhang <Fannie.Zhang@arm.com>
Run-TryBot: fannie zhang <Fannie.Zhang@arm.com>
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Reviewed-by: Cherry Zhang <cherryyz@google.com>