internal/runtime/maps: clean up put slot calls

internal/runtime/maps: clean up put slot calls

Use matchEmptyOrDeleted instead of matchEmpty.
Streamline the code a bit.
TODO: replicate in all the _fast files.
Change-Id: I4df16a13a19df3aaae0c42e0c12f20552f08ead6
Reviewed-on: https://go-review.googlesource.com/c/go/+/624055
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>