cmd/compile: define starting block before opening html writer

Opening the html writer can fail, and the failure printer wants
to use the entry block's line number. So make sure we set up
the entry block first.

Fixes #40919

Change-Id: I4ffa2839b45a721bbaf04ff84418e8108fa1cc37
Reviewed-on: https://go-review.googlesource.com/c/go/+/249497


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>