internal/poll: move isBlocking checks to inside execIO
This is a step towards deferring adding the handle to IOCP until the
first IO operation.
Having all isBlocking checks inside execIO will make it easier to delay
getting that information until the first IO operation.
It also makes the code simpler, as Pread and Pwrite now only modify
the overlapped object offset, not the one in the FD struct, so they
don't need to reverse the offset change after the IO operation.
For #76391
Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest,gotip-windows-amd64-race
Change-Id: Iea680c502c9417b9569a0b1a9da6f7e6bf916f6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/742283
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>