[dev.regabi] cmd/compile: tighten typecheckdef to *ir.Name

We only actually care about ir.Names in typecheckdef, so don't bother
calling it on anything else. Allows us to get rid of some more
superfluous .Name() calls and .(*ir.Name) assertions.

Passes toolstash -cmp.

Change-Id: I78c7cb680178991ea185958b47a36f101d4d5ef7
Reviewed-on: https://go-review.googlesource.com/c/go/+/281004


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>