cmd/compile: in poset, refactor aliasnode

In preparation for allowing to make multiple nodes as aliases
in a single pass, refactor aliasnode splitting out the case
in which one of the nodes is not in the post into a new
funciton (aliasnewnode).

No functional changes, passes toolstash -cmp

Change-Id: I19ca6ef8426f8aec9f2622b6151c5c617dbb25b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/200859Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/poset.go

@@ -407,7 +407,19 @@ func (po *poset) newconst(n *Value) {
 	po.upushconst(i, 0)
 }
 
-// aliasnode records that n2 is an alias of n1
+// aliasnewnode records that a single node n2 (not in the poset yet) is an alias
+// of the master node n1.
+func (po *poset) aliasnewnode(n1, n2 *Value) {
+	i1, i2 := po.values[n1.ID], po.values[n2.ID]
+	if i1 == 0 || i2 != 0 {
+		panic("aliasnewnode invalid arguments")
+	}
+
+	po.values[n2.ID] = i1
+	po.upushalias(n2.ID, 0)
+}
+
+// aliasnode records that n2 (already in the poset) is an alias of n1
 func (po *poset) aliasnode(n1, n2 *Value) {
 	i1 := po.values[n1.ID]
 	if i1 == 0 {
@@ -415,7 +427,9 @@ func (po *poset) aliasnode(n1, n2 *Value) {
 	}
 
 	i2 := po.values[n2.ID]
-	if i2 != 0 {
+	if i2 == 0 {
+		panic("aliasnode for non-existing node")
+	}
 	// Rename all references to i2 into i1
 	// (do not touch i1 itself, otherwise we can create useless self-loops)
 	for idx, n := range po.nodes {
@@ -452,12 +466,6 @@ func (po *poset) aliasnode(n1, n2 *Value) {
 		po.constants[val] = i1
 		po.upushconst(i1, i2)
 	}
-
-	} else {
-		// n2.ID wasn't seen before, so record it as alias to i1
-		po.values[n2.ID] = i1
-		po.upushalias(n2.ID, 0)
-	}
 }
 
 func (po *poset) isroot(r uint32) bool {
@@ -1093,11 +1101,11 @@ func (po *poset) SetEqual(n1, n2 *Value) bool {
 		i1 = po.newnode(n1)
 		po.roots = append(po.roots, i1)
 		po.upush(undoNewRoot, i1, 0)
-		po.aliasnode(n1, n2)
+		po.aliasnewnode(n1, n2)
 	case f1 && !f2:
-		po.aliasnode(n1, n2)
+		po.aliasnewnode(n1, n2)
 	case !f1 && f2:
-		po.aliasnode(n2, n1)
+		po.aliasnewnode(n2, n1)
 	case f1 && f2:
 		if i1 == i2 {
 			// Already aliased, ignore