cmd/compile/internal: better AST line highlight in ssa.html

We tend to set div class with the line number in HTML AST nodes. So that
the AST nodes can be highlighted with corresponding source and ssa ir.

The pure AST text dump is created first. And then it is parsed and
written to the HTML file.

CL 275785 changed the format of the line information in AST node dump,
which makes the HTMLWriter fail to parse the line information.

This CL updates the code in HTMLWriter to align with the format of AST
node dump.

Fix #48133

Change-Id: I2b56fc5e3e9771456d91f22caf23a427c235eb12
Reviewed-on: https://go-review.googlesource.com/c/go/+/347269


Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Than McIntosh <thanm@google.com>