cmd/compile: combine ssa.html columns with identical contents

Combine columns in ssa.html output if they are identical. There
can now be multiple titles per column which are all clickable to
expand and collapse their column. Give collapsed columns some
padding for better readability. Some of the work in this CL was
started by Josh Bleecher Snyder and mailed to me in order to
continue to completion.

Updates #37766

Change-Id: I313b0917dc1bafe1eb99d91798ea915e5bcfaae9
Reviewed-on: https://go-review.googlesource.com/c/go/+/226209


Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>