cmd/compile/internal/ssagen: sort locals by alignment, not size

cmd/compile/internal/ssagen: sort locals by alignment, not size

Sorting variables by size doesn't actually do anything, but sorting by
alignment makes sure we can optimally pack variables into the stack
frame.

While here, replace the monolithic description at the top of
cmpstackvarlt with line-by-line explanations of what each comparison
is doing.

Change-Id: I860677799618130ce4a55f084cec637cb9a2e295
Reviewed-on: https://go-review.googlesource.com/c/go/+/516197
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>