Add an optimization to remove ref temps when possible

This changeset makes inlineFunctions remove reference temps when possible. "Possible" is defined as any call that is a primitive or only contains primitive calls. While not perfect, this does remove ref temps for simple things like =, +=, *=, etc.

test_nested_var_iterator3 has been turned into a bug future in the pull request. A limitation in copy propagation was exposed by my changes, Tom is aware of this and said he will look into it. 
Is there any special reason you split up the creation of the AST nodes from their insertion? 

As in why change:
call->getStmtExpr()->insertBefore(new DefExpr(tmp));

to 

DefExpr* tmpDef = new DefExpr(tmp);
call->getStmtExpr()->insertBefore(tmpDef);