cmd/compile: remove reflectdata.{TypePtr,ITabAddr} wrappers

cmd/compile: remove reflectdata.{TypePtr,ITabAddr} wrappers

Remove these in favor of the explicit *At variants that take a
src.XPos.

Change-Id: I2c095b75e43b58fe31e3e1b15c811a66ac5a0f83
Reviewed-on: https://go-review.googlesource.com/c/go/+/518956
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>