There is currently no support for GOARCH=loong32, so the Optab.family
field is unused so far. Remove it to simplify the optab; the loong
assembler backend would likely already be overhauled into a sufficiently
different shape by the time we start to care for loong32, that the data
we have today would be useless anyway.

While at it, add a operand class slot for the 3rd source operand
(support for which will arrive in later commits), and rename the other
operand class fields to be self-documenting. The changes are being
merged into this patch for sake of reducing code churn.

Change-Id: Icf0988e34ff1c0f762c8e0708cfcef2e7954760c
Reviewed-on: https://go-review.googlesource.com/c/go/+/477715


Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Run-TryBot: Ben Shi <powerman1st@163.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Wayne Zuo <wdvxdr@golangcn.org>