cmd/compile: use cache in front of type assert runtime call

cmd/compile: use cache in front of type assert runtime call

That way we don't need to call into the runtime for every
type assertion (to an interface type).

name           old time/op  new time/op  delta
TypeAssert-24  3.78ns ± 3%  1.00ns ± 1%  -73.53%  (p=0.000 n=10+8)

Change-Id: I0ba308aaf0f24a5495b4e13c814d35af0c58bfde
Reviewed-on: https://go-review.googlesource.com/c/go/+/529316
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>