gc: fix order of evaluation

Pulling function calls out to happen before the
expression being evaluated was causing illegal
reorderings even without inlining; with inlining
it got worse.  This CL adds a separate ordering pass
to move things with a fixed order out of expressions
and into the statement sequence, where they will
not be reordered by walk.

Replaces lvd's CL 5534079.

Fixes #2740.

R=lvd
CC=golang-dev
https://golang.org/cl/5569062

test/fixedbugs/bug401.go

@@ -1,29 +1,46 @@
-// $G $D/$F.go || echo "Bug398"
+// $G $D/$F.go && $L $F.$A && ./$A.out || echo "Bug401"
 
 // Copyright 2011 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 // Issue 2582
-package foo
-    
-type T struct {}
+package main
+
+type T struct{}
+
 func (T) cplx() complex128 {
-	for false {}  // avoid inlining
-	return complex(1,0)
+	for false {
+	} // avoid inlining
+	return complex(1, 0)
+}
+
+func (T) cplx2() complex128 {
+	return complex(0, 1)
 }
 
 type I interface {
 	cplx() complex128
 }
 
-func f(e float32, t T) {
+func main() {
 
-    	_ = real(t.cplx())
-    	_ = imag(t.cplx())
+	var t T
+
+	if v := real(t.cplx()); v != 1 {
+		panic("not-inlined complex call failed")
+	}
+	_ = imag(t.cplx())
+
+	_ = real(t.cplx2())
+	if v := imag(t.cplx2()); v != 1 {
+		panic("potentially inlined complex call failed")
+	}
 
 	var i I
 	i = t
-    	_ = real(i.cplx())
-    	_ = imag(i.cplx())
-}
+	if v := real(i.cplx()); v != 1 {
+		panic("potentially inlined complex call failed")
+	}
+	_ = imag(i.cplx())
+}