cmd/compile: pull ssa OAPPEND expression handing into its own function

Pure code movement.

Change-Id: Ia07ee0b0041c931b08adf090f262a6f74a6fdb01
Reviewed-on: https://go-review.googlesource.com/21546
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -2066,6 +2066,16 @@ func (s *state) expr(n *Node) *ssa.Value {
 		return s.newValue1(ssa.OpGetG, n.Type, s.mem())
 
 	case OAPPEND:
+		return s.exprAppend(n)
+
+	default:
+		s.Unimplementedf("unhandled expr %s", opnames[n.Op])
+		return nil
+	}
+}
+
+// exprAppend converts an OAPPEND node n to an ssa.Value, adds it to s, and returns the Value.
+func (s *state) exprAppend(n *Node) *ssa.Value {
 	// append(s, e1, e2, e3).  Compile like:
 	// ptr,len,cap := s
 	// newlen := len + 3
@@ -2158,11 +2168,6 @@ func (s *state) expr(n *Node) *ssa.Value {
 	delete(s.vars, &ptrVar)
 	delete(s.vars, &capVar)
 	return s.newValue3(ssa.OpSliceMake, n.Type, p, nl, c)
-
-	default:
-		s.Unimplementedf("unhandled expr %s", opnames[n.Op])
-		return nil
-	}
 }
 
 // condBranch evaluates the boolean expression cond and branches to yes