The only place set releasetime to negative is in runtime.selectgo
(when blockprofilerate greater than zero), so we are safe in compiler
code.

But scasetype must keep in sync with runtime/select.go scase struct, so
releasetime must be int64.

Change-Id: I39ea944f5f2872452d3ffd57f7604d51e0d2590a
Reviewed-on: https://go-review.googlesource.com/c/go/+/179799
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>