[dev.regabi] cmd/compile: simplify ir.Func somewhat

Two simplifications:

1. Statements (including ODCLFUNC) don't have types, and the
Func.Nname already has a type. There's no need for a second one.
However, there is a lot of code that expects to be able to call
Func.Type, so leave a forwarding method, like with Sym and Linksym.

2. Inline and remove ir.NewFuncNameAt. It doesn't really save any
code, and it's only used a handful of places.

Passes toolstash -cmp.

Change-Id: I51acaa341897dae0fcdf2fa576a10174a2ae4d1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/280648


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>