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>

0 files changed, 0 insertions, 0 deletions