[AMDGPU] Partial revert for the ba447bae7448435c9986eece0811da1423972fdd

   "Divergence driven ISel. Assign register class for cross block values
       according to the divergence."
       that discovered the design flaw leading to several issues that
       required to be solved before.

       This change reverts AMDGPU specific changes and keeps common part
       unaffected.

llvm-svn: 362749