cmd/compile: improve CFG size in ssa.html

This change forces CFGs to take the full width of their column
and allows them to be as tall as necessary.

In my (recent) experience, this makes them far less likely to
be cropped, which makes them much more useful.

On rare occasions, this can lead to gigantic CFGs,
but if you've bothered to explicitly request a CFG,
this is still better than an irrevocably truncated CFG.

Change-Id: I9a649ea57fa3c2792998bb71331a2580e429b36a
Reviewed-on: https://go-review.googlesource.com/c/go/+/222618


Reviewed-by: Cherry Zhang <cherryyz@google.com>