[ConstraintElim] Queue facts and checks directly.

[ConstraintElim] Queue facts and checks directly.

This allows interleaving facts and checks in a single block. In
particular this enables using facts from assumes for conditions in the
same block that come after the assume.

This could be extended to only try to simplify checks at the point where
a condition is used.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D138452