cmd/compile: use typeAndStr directly in signatslice

Currently, we store *types.Type in signatslice, then convert it to
typeAndStr during write runtime type process.

Instead, we can just store typeAndStr directly in signatslice when
adding type to signatset. Not a big win, but simplify the code a bit.

Change-Id: Ie1c8cfa5141da32b6ec3ce5844ba150d2765fe90
Reviewed-on: https://go-review.googlesource.com/c/go/+/343529


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>