[AMDGPU] Translate reqd_work_group_size into amdgpu_flat_work_group_size

These two attributes specify the same info in a different way.
AMGPU BE only checks the latter as a target specific attribute
as opposed to language specific reqd_work_group_size.

This change produces amdgpu_flat_work_group_size out of
reqd_work_group_size if specified.

Differential Revision: https://reviews.llvm.org/D31728

llvm-svn: 299678