Check bool attribute value in getOptionalBoolLoopAttribute.

Summary:
Check the bool value of the attribute in getOptionalBoolLoopAttribute
not just its existance.
Eliminates the warning noise generated when vectorization is explicitly disabled.

Reviewers: Meinersbur, hfinkel, dmgreen

Subscribers: jlebar, sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D57260

llvm-svn: 352555