cmd/compile: use typeAndStr directly in signatslice
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 <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go