Add a marks[] array, initialize members to false, and add a test to ensure marks[] is same size as exprs[]
Update buildBasicBlocks() and append() to pass the mark. Force mark to TRUE for CondStmt and Loop conditional expressions
Merge pull request #907 from noakesmichael/bb-2

Introduce isEssential flag

Add marks[]  to each BasicBlock.

Update buildBasicBlocks to set this mark to true for the conditional expression
of While/For loops and CondStmt

Compiled: STD clang/darwin, STD gcc/linux64, LLVM gcc/linux64
Tested: paratest --verify
Review: None