R600/SI: fix and cleanup SI register definition v2

Prevent producing real strange tablegen code by using
proper register sizes, alignments and hierarchy.

Also cleanup the unused definitions and add some comments.

v2: add SGPR 512 bit registers, stop registers from wrapping around,
    fix SGPR alignment

This is a candidate for the mesa-stable branch.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
llvm-svn: 176098