runtime: add explicit lower bounds check to decoderune

decoderune is only called by generated code, so we can guarantee that
it's non-negative. This allows eliminating the automatic bounds check in
it.

To make this work, we need to expand the existing optimization to uints.
This generally enables BCE for cases like this:
```go
func test(list []int, idx uint64) int {
        if idx >= uint64(len(list)) {
                return 0
        }

        list1 := list[idx:]
        return list1[0]
}
```

Change-Id: I86a51b26ca0e63522dec99f7d6efe6bdcd2d6487
GitHub-Last-Rev: 82d44e0a080b53ee02c31ee1f92a8a0acd8d2621
GitHub-Pull-Request: golang/go#76610
Reviewed-on: https://go-review.googlesource.com/c/go/+/725101
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/runtime/utf8.go b/src/runtime/utf8.go
index 52b757662d..e94b89ef3e 100644
--- a/src/runtime/utf8.go
+++ b/src/runtime/utf8.go
@@ -57,10 +57,10 @@ func countrunes(s string) int {
 // If the string appears to be incomplete or decoding problems
 // are encountered (runeerror, k + 1) is returned to ensure
 // progress when decoderune is used to iterate over a string.
-func decoderune(s string, k int) (r rune, pos int) {
+func decoderune(s string, k uint) (r rune, pos uint) {
 	pos = k
 
-	if k >= len(s) {
+	if k >= uint(len(s)) {
 		return runeError, k + 1
 	}