The current code delays the literal pool until the very last moment,
but based on the assumption that span-dependent jumps are as
short as possible. If they need to be enlarged in a later round, that
very last moment may be too late. Flush a little early to prevent that.

Fixes #13579.

Change-Id: I759b5db5c43a977bf2b940872870cbbc436ad141
Reviewed-on: https://go-review.googlesource.com/18972Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Russ Cox <rsc@golang.org>