[WarnMissedTransforms] Set default to 1.

Summary:
Set default value for retrieved attributes to 1, since the check is against 1.
Eliminates the warning noise generated when the attributes are not present.

Reviewers: sanjoy

Subscribers: jlebar, llvm-commits

Differential Revision: https://reviews.llvm.org/D57253

llvm-svn: 352238