Bug #9439: Ensure that stage 0 compiler isn't affected

Summary:
Bug #9439 will cause miscompilation of GHC's LLVM backend. Here we
ensure that an affected compiler isn't used to bootstrap.

Test Plan: Attempt to bootstrap GHC with an affected stage 0 compiler.

Reviewers: rwbarton, austin

Subscribers: simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D159