go spec: handle a corner case of a special case for shifts...

- Added some additional examples.
- 6g appears to implement this semantics already.

Fixes #658.

R=rsc, r, iant, ken
CC=golang-dev
https://golang.org/cl/4538119

doc/go_spec.html

 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of May 24, 2011 -->
+<!-- subtitle Version of June 7, 2011 -->
 
 <!--
 TODO
@@ -2720,7 +2720,9 @@ The right operand in a shift expression must have unsigned integer type
 or be an untyped constant that can be converted to unsigned integer type.
 If the left operand of a non-constant shift expression is an untyped constant,
 the type of the constant is what it would be if the shift expression were
-replaced by its left operand alone.
+replaced by its left operand alone; the type is <code>int</code> if it cannot
+be determined from the context (for instance, if the shift expression is an
+operand in a comparison against an untyped constant).
 </p>
 
 <pre>
@@ -2729,6 +2731,9 @@ var i = 1&lt;&lt;s           // 1 has type int
 var j int32 = 1&lt;&lt;s     // 1 has type int32; j == 0
 var k = uint64(1&lt;&lt;s)   // 1 has type uint64; k == 1&lt;&lt;33
 var m int = 1.0&lt;&lt;s     // legal: 1.0 has type int
+var n = 1.0&lt;&lt;s != 0    // legal: 1.0 has type int; n == false if ints are 32bits in size
+var o = 1&lt;&lt;s == 2&lt;&lt;s   // legal: 1 and 2 have type int; o == true if ints are 32bits in size
+var p = 1&lt;&lt;s == 1&lt;&lt;33  // illegal if ints are 32bits in size: 1 has type int, but 1&lt;&lt;33 overflows int
 var u = 1.0&lt;&lt;s         // illegal: 1.0 has type float64, cannot shift
 var v float32 = 1&lt;&lt;s   // illegal: 1 has type float32, cannot shift
 var w int64 = 1.0&lt;&lt;33  // legal: 1.0&lt;&lt;33 is a constant shift expression