radeonsi: increase array size for shader inputs and outputs

radeonsi: increase array size for shader inputs and outputs

and add assertions to prevent buffer overflow. This fixes corruption
of the si_shader struct.

NOTE: This is a candidate for the 9.1 branch.

[ Cherry-pick of r600g commit da33f9b919039442e9ab51f9b1d1c83a73607133 ]

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