cmd/compile: display AST IR in ssa.html

This change adds a new column, AST IR. That column contains
nodes for a function specified in $GOSSAFUNC.

Also this CL enables horizontal scrolling of sources and AST columns.

Fixes #26662

Change-Id: I3fba39fd998bb05e9c93038e8ec2384c69613b24
Reviewed-on: https://go-review.googlesource.com/126858


Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>