Revert "[dataflow] use true/false literals in formulas, rather than variables"

This reverts commit 36bd5bd8.

This change is causing a test failure on several build bots:
- https://lab.llvm.org/buildbot/#/builders/139/builds/50255
- https://lab.llvm.org/buildbot/#/builders/216/builds/27735
- https://lab.llvm.org/buildbot/#/builders/247/builds/9334