Consumed analysis: improve handling of conditionals.

Patch by chris.wailes@gmail.com.

* The TestedVarsVisitor was folded into the ConsumedStmtVisitor.
* The VarTestResult class was updated to allow these changes.
* The PropagationInfo class was updated for the same reasons.
* Correctly handle short-circuiting of Boolean operations.
* Blocks are now marked as unreachable when we can statically prove we will
  never branch to them.
* Unreachable blocks are skipped by the analysis.

llvm-svn: 189594