cmd/compile/internal/typecheck: replace Temp calls with TempAt

cmd/compile/internal/typecheck: replace Temp calls with TempAt

Steps towards eliminating implicit dependencies on base.Pos and
ir.CurFunc. Mechanical CL produced with gofmt -r.

Change-Id: I070015513cb955cbe87f9a148d81db8c0d4b0dc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/520605
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>