cmd/compile: use fuse to implement shortcircuit loop

The rewrite loop in shortcircuit is identical to the one in fuse.
That's not surprising; shortcircuit is fuse-like.

Take advantage of that by merging the two loops.

Passes toolstash-check.

Change-Id: I642cb39a23d2ac8964ed577678f062fce721439c
Reviewed-on: https://go-review.googlesource.com/c/go/+/229003
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -20,6 +20,7 @@ const (
 	fuseTypePlain fuseType = 1 << iota
 	fuseTypeIf
 	fuseTypeIntInRange
+	fuseTypeShortCircuit
 )
 
 // fuse simplifies control flow by joining basic blocks.
@@ -38,6 +39,9 @@ func fuse(f *Func, typ fuseType) {
 			if typ&fuseTypePlain != 0 {
 				changed = fuseBlockPlain(b) || changed
 			}
+			if typ&fuseTypeShortCircuit != 0 {
+				changed = shortcircuitBlock(b) || changed
+			}
 		}
 		if changed {
 			f.invalidateCFG()

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

@@ -58,20 +58,7 @@ func shortcircuit(f *Func) {
 	//   if v goto t else u
 	// We can redirect p to go directly to t instead of b.
 	// (If v is not live after b).
-	for changed := true; changed; {
-		changed = false
-		for i := len(f.Blocks) - 1; i >= 0; i-- {
-			b := f.Blocks[i]
-			if fuseBlockPlain(b) {
-				changed = true
-				continue
-			}
-			changed = shortcircuitBlock(b) || changed
-		}
-		if changed {
-			f.invalidateCFG()
-		}
-	}
+	fuse(f, fuseTypePlain|fuseTypeShortCircuit)
 }
 
 // shortcircuitBlock checks for a CFG in which an If block