cmd/compile: track funcsyms by ir.Name instead of types.Sym

This is a cleanup to bring funcsym tracking a little closer to the
ir.Func. (I thought I needed this for a later change. That turned out
not to be the case, but it's a nice cleanup.)

Change-Id: I53e692f5d7ba4be56d42d8e0aefc06284cea0661
Reviewed-on: https://go-review.googlesource.com/c/go/+/305270


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>