Propagate Undef in llvm.cos Intrinsic

Summary:
The llvm cos intrinsic currently does not propagate undef's. This change
transforms cos(undef) to null value or 0.

There are 2 test cases added as well.

Patch by Anna Thomas!

Reviewers: sanjoy

Subscribers: majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D18863

llvm-svn: 265825