internal/poll: consolidate cancelIO logic into waitIO

internal/poll: consolidate cancelIO logic into waitIO

This is a step towards deferring adding the handle to IOCP until the
first IO operation.

The main goal of this CL is to remove the fd.pollable() check in
cancelIO.

For #76391

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest,gotip-windows-amd64-race
Change-Id: I76263ce12980297d88a5f6c514e4074dfee428cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/740540
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Damien Neil <dneil@google.com>