[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>

src/cmd/compile/internal/ir/sizeof_test.go

@@ -21,9 +21,8 @@ func TestSizeof(t *testing.T) {
 		_64bit uintptr     // size on 64bit platforms
 	}{
 		{Func{}, 152, 280},
-		{Name{}, 36, 64},
-		{Param{}, 44, 88},
-		{node{}, 88, 152},
+		{Name{}, 132, 232},
+		{node{}, 84, 144},
 	}
 
 	for _, tt := range tests {