Move the shared code into byteal.memeqbody. This will allow to implement
optimizations (e.g. for #29001) in a single function.

Change-Id: Iaa34ddeb7068d92c35a8b4e581b7fd92da56535c
Reviewed-on: https://go-review.googlesource.com/c/go/+/166677
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>