cmd/compile: give informative error instead of "stupid shift"

Fixes #13940.

Change-Id: I00fe377c949e5be4cbc035f6ca18e547e326bfba
Reviewed-on: https://go-review.googlesource.com/19856
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/mparith2.go

@@ -217,7 +217,11 @@ func mplshfixfix(a, b *Mpint) {
 
 	s := Mpgetfix(b)
 	if s < 0 || s >= Mpprec {
-		Yyerror("stupid shift: %d", s)
+		msg := "shift count too large"
+		if s < 0 {
+			msg = "invalid negative shift count"
+		}
+		Yyerror("%s: %d", msg, s)
 		Mpmovecfix(a, 0)
 		return
 	}
@@ -236,7 +240,7 @@ func mprshfixfix(a, b *Mpint) {
 
 	s := Mpgetfix(b)
 	if s < 0 {
-		Yyerror("stupid shift: %d", s)
+		Yyerror("invalid negative shift count: %d", s)
 		if a.Val.Sign() < 0 {
 			Mpmovecfix(a, -1)
 		} else {

test/fixedbugs/bug108.go

@@ -6,6 +6,6 @@
 
 package main
 func f() {
-	v := 1 << 1025;		// ERROR "overflow|stupid shift"
+	v := 1 << 1025;		// ERROR "overflow|shift count too large"
 	_ = v
 }