[dev.regabi] cmd/compile: adjust IR representations

Based on actually using the IR when prototyping adding
type assertions, a few changes to improve it:

- Merge DeferStmt and GoStmt, since they are variants of one thing.
- Introduce LogicalExpr for && and ||, since they (alone) need an init list before Y.
- Add an explicit op to various constructors to make them easier to use.
- Add separate StructKeyExpr - it stores Value in a different abstract location (Left) than KeyExpr (Right).
- Export all fields for use by rewrites (and later reflection).

Passes buildall w/ toolstash -cmp.

Change-Id: Iefbff2386d2bb9ef511ce53b7f92ff6c709dc991
Reviewed-on: https://go-review.googlesource.com/c/go/+/275883


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>