[AMDGPU] Support for "uniform-work-group-size" attribute

Updated the annotate-kernel-features pass to support the propagation of uniform-work-group attribute from the kernel to the called functions. Once this pass is run, all kernels, even the ones which initially did not have the attribute, will be able to indicate weather or not they have uniform work group size depending on the value of the attribute. 

Differential Revision: https://reviews.llvm.org/D50200

llvm-svn: 348971