Introduce the predicate expr::isStmtExpr()
Exploit the new isStmtExpr() in deadExpressionElimination
Introduce isInCForLoopHeader and use this specialized version in deadExpressionElimination
Merge pull request #859 from noakesmichael/StmtExpr

Add a predicate to determine if an expression is a "Statement Expression" and apply
the predicate in deadExpressionElimination (which is itself a short-hand for 
deadStatementExpressionElimination).  Convert deadExpressionElimination to use
a specialized version of the code to check for expressions in LoopHeaders; it only
needs to check for expressions in CForLoop headers.

Compiled: STD Clang/Darwin, STD GCC/Linux64
Tested: Full STD regression suite
Review: None