[dev.regabi] cmd/compile: simplify FuncName and PkgFuncName

Now that we have proper types, these functions can be restricted to
only allowing *ir.Func, rather than any ir.Node. And even more
fortunately, all of their callers already happen to always
pass *ir.Func arguments, making this CL pretty simple.

Passes toolstash -cmp.

Change-Id: I21ecd4c8cee3ccb8ba86b17cedb2e71c56ffe87a
Reviewed-on: https://go-review.googlesource.com/c/go/+/280440


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>