[dev.regabi] cmd/compile: make ir.Name the ONAME Node implementation

[dev.regabi] cmd/compile: make ir.Name the ONAME Node implementation

Before this CL, an ONAME Node was represented by three structs
linked together: a node, a Name, and a Param. Previous CLs removed
OLABEL and OPACK from the set of nodes that knew about Name.
Now Name can be repurposed to *be* the ONAME Node implementation,
replacing three linked structs totaling 152+64+88 = 304 bytes (64-bit)
with a single 232-byte struct.

Many expressions in the code become simpler as well, without having
to use .Param. and sometimes even .Name().
(For a node n where n.Name() != nil, n.Name() == n.(*Name) now.)

Passes buildall w/ toolstash -cmp.

Change-Id: Ie719f1285c05623b9fd2faaa059e5b360a64b3be
Reviewed-on: https://go-review.googlesource.com/c/go/+/274094
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>