AMDGPU: Implicit kernel arguments related optimization when uniform-workgroup-size=true

 Summary:
   Under code object version 5, ockl_get_local_size returns the value computed by the expression:
workgroup_id < hidden_block_count ? hidden_group_size : hidden_remainder
For functions with the attribute uniform-work-group-size=true. we can evaluate workgroup_id < hidden_block_count
as true, and thus hidden_group_size is returned for ockl_get_local_size.
  With uniform-workgroup-size=true, this work also set all remainders to zero, and if there
is reqd_work_group_size, we also set work-group-size to the required value from the metadata.

Reviewers:
  arsenm and bcahoon

Differential Revision:
  https://reviews.llvm.org/D131276

Author: changpeng