[dev.typeparams] cmd/compile: simplify openDeferSave

Now it is only used to save the deferred the function (closure),
which must be a function type. Simplify the code.

Change-Id: Id4b8f2760fbf39a95883df2327f97378e7edab88
Reviewed-on: https://go-review.googlesource.com/c/go/+/326060


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>