Checkpoint some minor cleanup
Merge pull request #679 from noakesmichael/loopCleanup-2

Continued refactoring for loops

Introduced a simple virtual method, currently on BlockStmt, to support dead code elimination.
Existing code relied on inlined code to inspect the blockInfo field of the BlockStmt and it did not
seem much better to merely wrap this with accessors on Loops.

Compiled STD Clang on Mac, STD GCC on Linux, LLVM GCC on Linux
Tested Full paratest STD --verify and LLVM --verify
Review: None