[dev.typeparams] cmd/compile: calculate variable sizes in walk

Walk already explicitly calculates the size of all expression types,
to make sure they're known before SSA generation (which is concurrent,
and thus not safe to modify shared state like types). Might as well
compute all local variable sizes too, to be consistent.

Reduces the burden of the frontend to make sure it's calculated the
size of types that only the backend cares about.

Passes toolstash -cmp.

Change-Id: I68bcca67b4640bfc875467e4ed4d47104b1932f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/282912


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