cmd/compile: prevent duplicated works in WriteRuntimeTypes

While processing signatset, the entry is deleted immediately after being
pushed to signatslice. Then calling writeType may add the same type
to signatset again. That would add more works, though not a big impact
to the performace, since when writeType is guarded by s.Siggen() check.

Instead, we should keep the entry in signatset, so written type will
never be added again.

This change does not affect compiler performace, but help debugging
issue like one in #46386 easier.

Change-Id: Iddafe773885fa21cb7003ba27ddf9554fc3f297d
Reviewed-on: https://go-review.googlesource.com/c/go/+/326029


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>