code review 157082: Permit omission of hi bound in slices.

src/pkg/go/printer/testdata/expressions.input

 // Copyright 2009 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.
 
 package expressions
 
 type T struct {
         x, y, z int
 }
 
@@ skipping 36 matching lines @@
         _ = a - ^b;
         _ = a / *p;
         _ = a + b*c;
         _ = 1 + b*c;
         _ = a + 2*c;
         _ = a + c*2;
         _ = 1 + 2*3;
         _ = s[1 : 2*3];
         _ = s[a : b-c];
         _ = s[0:];
+        _ = s[a+b];
         _ = s[a+b :];
+        _ = a[a<<b+1];
+        _ = a[a<<b+1 :];
         _ = s[a+b : len(s)];
         _ = s[len(s) : -a];
         _ = s[a : len(s)+1];
         _ = s[a : len(s)+1]+s;
 
         // spaces around operators with equal or lower precedence than comparisons
         _ = a == b;
         _ = a != b;
         _ = a > b;
         _ = a >= b;
@@ skipping 257 matching lines @@
 
 func addState(s []state, inst instr, match []int) {
         // handle comments correctly in multi-line expressions
         for i := 0; i < l; i++ {
                 if s[i].inst.index() == index && // same instruction
                    s[i].match[0] < pos {        // earlier match already going; leftmost wins
                         return s
                  }
         }
 }