go/types: nicer shift error message

Updates #13940.

Change-Id: I41974c292dd981d82ac03b9b8b406713445362c3
Reviewed-on: https://go-review.googlesource.com/20081
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/types/expr.go

@@ -660,10 +660,10 @@ func (check *Checker) shift(x, y *operand, e *ast.BinaryExpr, op token.Token) {
 				return
 			}
 			// rhs must be within reasonable bounds
-			const stupidShift = 1023 - 1 + 52 // so we can express smallestFloat64
+			const shiftBound = 1023 - 1 + 52 // so we can express smallestFloat64
 			s, ok := constant.Uint64Val(yval)
-			if !ok || s > stupidShift {
-				check.invalidOp(y.pos(), "stupid shift count %s", y)
+			if !ok || s > shiftBound {
+				check.invalidOp(y.pos(), "invalid shift count %s", y)
 				x.mode = invalid
 				return
 			}

src/go/types/testdata/shifts.src

@@ -10,8 +10,8 @@ func shifts0() {
 		s = 10
 		_ = 0<<0
 		_ = 1<<s
-		_ = 1<<- /* ERROR "stupid shift" */ 1
-		_ = 1<<1075 /* ERROR "stupid shift" */
+		_ = 1<<- /* ERROR "invalid shift" */ 1
+		_ = 1<<1075 /* ERROR "invalid shift" */
 		_ = 2.0<<1
 
 		_ int = 2<<s