Iterators: Revise the test for determining if a SymExpr is in a loop header to handle new representation
Merge pull request #954 from noakesmichael/baselineFix

The code for lowering iterators included a subtle check to determine whether
a symExpr is contained in the header of a loop.  Unlike similar tests it relied
on the overall shape of the code above the SymExpr rather than the more
common reliance on the call-type of the blockInfo.

For the short term I have updated the test to (a) match the new
shape (b) be slightly more robust (c) added a tagged comment
so that I can come back and improve this test when the Loop
work is more sound.

Compiled: STD clang/darwin, STD gcc/linux64, LLVM gcc/linux64
Tested: paratest --baseline and standard paratest
Review: None