cmd/compile: add types.RecalcSize

This is the only case where Align is assigned outside of package
types. Rather than adding a SetAlign method, adding a RecalcSize
function is a bit more descriptive.

Change-Id: I1b3c01ebd0e41183665baa63c926592865bbbd0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/345479


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