cmd/compile/internal/ir: remove OSIZEOF, etc

cmd/compile/internal/ir: remove OSIZEOF, etc

These are no longer needed after the previous CL, which moved handling
of unsafe.Sizeof, etc. directly into the unified frontend.

Change-Id: Ieb35ffca0bc25319e58132fb5d035c5b441acea7
Reviewed-on: https://go-review.googlesource.com/c/go/+/527098
Auto-Submit: Matthew Dempsky <mdempsky@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>
Reviewed-by: Keith Randall <khr@google.com>