cmd/compile/internal/ir: remove NilExpr.Sym_

cmd/compile/internal/ir: remove NilExpr.Sym_

Historically, the compiler used to use a name node to represent "nil".
Now, "nil" is represented by NilExpr, so it's not necessary to associate
a Sym with it anymore.

Passes toolstash-check.

Change-Id: Ied1ddefa06ea55ada18ca52c8fcf71defa4c23b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/406174
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>