[dev.regabi] cmd/compile: remove Node.{,Set}Walkdef

After the previous commit, we no longer access Walkdef on anything but
ir.Names, so we can remove them from the Node interface and miniNode.

The flag bits storage should also move from miniNode.bits to
Name.flags, but the latter is already full at the moment. Leaving as a
TODO for now.

Passes toolstash -cmp.

Change-Id: I2427e4cf7bc68dc1d1529f40fb93dd9f7a9149f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/281005


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>