[dev.regabi] cmd/compile: remove TypeAssertExpr {Src,Dst}Type fields

CL 283233 added reflectType method to ssagen.state, which we can use to
setup type address in the SSA backend in favor of the frontend. However,
this will change the order of symbols generation, so not safe for toolstash.

Change-Id: Ib6932ec42a9d28c3fd7a1c055596e75494c29843
Reviewed-on: https://go-review.googlesource.com/c/go/+/284115


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: Matthew Dempsky <mdempsky@google.com>