cmd/compile/internal/noder: handle unsafe.Sizeof, etc in unified IR

Previously, the unified frontend implemented unsafe.Sizeof, etc that
involved derived types by constructing a normal OSIZEOF, etc
expression, including fully instantiating their argument. (When
unsafe.Sizeof is applied to a non-generic type, types2 handles
constant folding it.)

This worked, but involves unnecessary work, since all we really need
to track is the argument type (and the field selections, for
unsafe.Offsetof).

Further, the argument expression could generate temporary variables,
which would then go unused after typecheck replaced the OSIZEOF
expression with an OLITERAL. This results in compiler failures after
CL 523315, which made later passes stricter about expecting the
frontend to not construct unused temporaries.

Fixes #62515.

Change-Id: I37baed048fd2e35648c59243f66c97c24413aa94
Reviewed-on: https://go-review.googlesource.com/c/go/+/527097
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>

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