[CUDA] Add -fcuda-flush-denormals-to-zero.

Summary:
Setting this flag causes all functions are annotated with the
"nvvm-f32ftz" = "true" attribute.

In addition, we annotate the module with "nvvm-reflect-ftz" set
to 0 or 1, depending on whether -cuda-flush-denormals-to-zero is set.
This is read by the NVVMReflect pass.

Reviewers: tra, rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18671

llvm-svn: 265435