spec: clarify restrictions on RHS of non-constant shifts

For non-constant shifts with an untyped constant shift count, the
spec only said that it must "be converted to unsigned integer type".
go/types accepts any (arbitrarily large) integer value. Both cmd/compile
and gccgo require that the shift count be representable as a uint value
in that case (if the shift count is typed, it may be any unsigned integer
type).

This change adjusts the spec to state what the compilers have been doing
all along. The new wording matches similar rules elsewhere (e.g., for
untyped array and slice indices). Also, while technically this is a
restriction (we could permit arbitrarily large shift counts), in practice
this is irrelevant.

Fixes #14822.

Change-Id: Ia75834c67483cf761c10025c8df758f225ef67c2
Reviewed-on: https://go-review.googlesource.com/45072Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 9, 2017",
+	"Subtitle": "Version of June 7, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -3384,7 +3384,7 @@ to the type of the other operand.
 
 <p>
 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.
+or be an untyped constant representable by a value of type <code>uint</code>.
 If the left operand of a non-constant shift expression is an untyped constant,
 it is first converted to the type it would assume if the shift expression were
 replaced by its left operand alone.