cmd/compile: factor out access to thisT

cmd/compile: factor out access to thisT

isifacemethod accessed thisT without checking if it was initialized,
opening the possibility for a bug during type checking. Give better
name, move it to package types, and provide accessor instead.

Change-Id: I29ffc408252a4ba4ef1de218fa154397786c9be6
Reviewed-on: https://go-review.googlesource.com/41673
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>