cmd/compile/internal/types2: print "nil" rather than "untyped nil"

When we have a typed nil, we already say so; thus it is sufficient
to use "nil" in all the other cases.

This is closer to (1.17) compiler behavior. In cases where the
1.17 compiler prints "untyped nil" (e.g., wrong uses of "copy"),
we already print a different message. We can do better in those
cases as well; will be addressed in a separate CL (see #49735).

Fixes #48852.

Change-Id: I9a7a72e0f99185b00f80040c5510a693b1ea80f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/366276


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>