cmd/compile: fix wrong field type in scasetype

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>