[dev.ssa] cmd/compile: fix -N build

The OpSB hack didn't quite work.  We need to really
CSE these ops to make regalloc happy.

Change-Id: I9f4d7bfb0929407c84ee60c9e25ff0c0fbea84af
Reviewed-on: https://go-review.googlesource.com/19083
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -10,6 +10,34 @@ import "sort"
 // Values are just relinked, nothing is deleted.  A subsequent deadcode
 // pass is required to actually remove duplicate expressions.
 func cse(f *Func) {
+	if !f.Config.optimize {
+		// Don't do CSE in this case.  But we need to do
+		// just a little bit, to combine multiple OpSB ops.
+		// Regalloc gets very confused otherwise.
+		var sb *Value
+	outer:
+		for _, b := range f.Blocks {
+			for _, v := range b.Values {
+				if v.Op == OpSB {
+					sb = v
+					break outer
+				}
+			}
+		}
+		if sb == nil {
+			return
+		}
+		for _, b := range f.Blocks {
+			for _, v := range b.Values {
+				for i, a := range v.Args {
+					if a.Op == OpSB {
+						v.Args[i] = sb
+					}
+				}
+			}
+		}
+		return
+	}
 	// Two values are equivalent if they satisfy the following definition:
 	// equivalent(v, w):
 	//   v.op == w.op