Don't refer to blocks in debug info when -g1

-g1 removes block information, but it turns out that procs can
refer to block information through parents.
Note [Splitting DebugBlocks] explains the parentage relationship.

Test Plan:
* ./validate
* added a new test

Reviewers: bgamari, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14894

Differential Revision: https://phabricator.haskell.org/D4496

(cherry picked from commit 0cbb13b3)