[dev.typeparams] cmd/compile/internal/types2: adjust unsafe.Alignof/Offsetof/Sizeof

Changed the implementation such that the result is a variable rather than
a constant if the argument type (or the struct in case of unsafe.Offsetof)
has a size that depends on type parameters.

Minor unrelated adjustments.

For #40301.

Change-Id: I1e988f1479b95648ad95a455c764ead829d75749
Reviewed-on: https://go-review.googlesource.com/c/go/+/335413


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>