cmd/compile: call phiElimValue from removePhiArg

With the exception of the shortcircuit pass, removePhiArg is always unconditionally followed by phiElimValue.
Move the phiElimValue inside removePhiArg.

Resolves a TODO.

See CL 357964 for more info.

Change-Id: I8460b35864f4cd7301ba86fc3dce08ec8041da7f
Reviewed-on: https://go-review.googlesource.com/c/go/+/465435
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Jakub Ciolek <jakub@ciolek.dev>

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

@@ -341,7 +341,7 @@ func (b *Block) swapSuccessors() {
 //	if v.Op != OpPhi {
 //	    continue
 //	}
-//	b.removeArg(v, i)
+//	b.removePhiArg(v, i)
 //
 // }
 func (b *Block) removePhiArg(phi *Value, i int) {
@@ -353,6 +353,7 @@ func (b *Block) removePhiArg(phi *Value, i int) {
 	phi.Args[i] = phi.Args[n]
 	phi.Args[n] = nil
 	phi.Args = phi.Args[:n]
+	phielimValue(phi)
 }
 
 // LackingPos indicates whether b is a block whose position should be inherited

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

@@ -96,11 +96,6 @@ func critical(f *Func) {
 
 				// splitting occasionally leads to a phi having
 				// a single argument (occurs with -N)
-				// TODO(cuonglm,khr): replace this with phielimValue, and
-				//                    make removePhiArg incorporates that.
-				if len(b.Preds) == 1 {
-					phi.Op = OpCopy
-				}
 				// Don't increment i in this case because we moved
 				// an unprocessed predecessor down into slot i.
 			} else {

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

@@ -330,7 +330,6 @@ func (b *Block) removeEdge(i int) {
 			continue
 		}
 		c.removePhiArg(v, j)
-		phielimValue(v)
 		// Note: this is trickier than it looks. Replacing
 		// a Phi with a Copy can in general cause problems because
 		// Phi and Copy don't have exactly the same semantics.

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

@@ -82,7 +82,6 @@ func fuseBranchRedirect(f *Func) bool {
 						continue
 					}
 					b.removePhiArg(v, k)
-					phielimValue(v)
 				}
 				// Fix up child to have one more predecessor.
 				child.Preds = append(child.Preds, Edge{p, pk.i})