runtime: add explicit lower bounds check to decoderune

By doing a full bounds check here, we eliminate the auto generated one,
which makes the codegen slightly better.

Requires I39ba2a18cbabc0559924d4d226dcb99dbe9a06ed for BCE, although
this can be merged before it.

Author: uellenberg

src/runtime/utf8.go

@@ -60,7 +60,7 @@ func countrunes(s string) int {
 func decoderune(s string, k int) (r rune, pos int) {
 	pos = k
 
-	if k >= len(s) {
+	if k < 0 || k >= len(s) {
 		return runeError, k + 1
 	}