cmd/compile: fix misalignment in sources column of generated ssa.html

cmd/compile: fix misalignment in sources column of generated ssa.html

Fix regression where line numbers in the sources column of generated
ssa.html output became misaligned with the source code. This was due
to some new margins applied to certain h2 elements during the work
to combine identical columns.

Fixes #38612

Change-Id: I067ccbfa30d5de5be29aab9863bc1e21f6ded128
Reviewed-on: https://go-review.googlesource.com/c/go/+/229766
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>