Do not insert a nil check before the call to a destructor method
Refine the comments
Merge pull request #783 from noakesmichael/nil-check

Fix a --baseline regression.

The Chapel Spec asserts that it is permitted to delete "nil".  However the NIL-checks are
inserted before every member access/call including the call to a destructor.  Add special
case code to catch this case.

Compiled  STD clang/darwin  STD/LLVM gcc/linux64
Tested: Fixed the --baseline and full paratest
Reviewed: Tom