AMDGPU: Align the implicit kernel argument segment to 8 bytes for v5

Summary:
  In emitting metadata for implicit kernel arguments, we need to be in sync with the actual loads
to align the implicit kernel argument segment to 8 byte boundary. In this work, we simply force
this alignment through the first implicit argument.
In addition, we don't emit metadata for any implicit kernel argument if none of them is actually used.

Reviewers: arsenm, b-sumner

Differential Revision: https://reviews.llvm.org/D123346