R600/SI: implement indirect adressing for SI

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177277 91177308-0d34-0410-b5e6-96231b3b80d8