r600g: Use LOOP_START_DX10 for loops

r600g: Use LOOP_START_DX10 for loops

LOOP_START_DX10 ignores the LOOP_CONFIG* registers, so it is not limited
to 4096 iterations like the other LOOP_* instructions.  Compute shaders
need to use this instruction, and since we aren't optimizing loops with
the LOOP_CONFIG* registers for pixel and vertex shaders, it seems like
we should just use it for everything.

Reviewed-by: Marek Olšák <maraeo@gmail.com>