Delete unreachable path in inner loop of DCE isEssential
Copy bulk of isEssential code from DCE to buildBasicBlocks.  Add printf's to DCE to confirm correct handling in BB.
Remove the verification printing from the inner loop of DCE
Merge pull request #910 from noakesmichael/bb-4

Transferred most of isEssential from deadCodeElimination() to BasicBlock construction

Compiled STD clang/darwin, STD gcc/linux64, LLVM gcc/linux64
Tested paratest, paratest --llvm --verify
Reviewed: None