cmd/compile/internal/ir: add typ parameter to NewNameAt

cmd/compile/internal/ir: add typ parameter to NewNameAt

Start making progress towards constructing IR with proper types.

Change-Id: Iad32c1cf60f30ceb8e07c31c8871b115570ac3bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/520263
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>