radeonsi: support indirect indexing in INTERP_* opcodes

radeonsi: support indirect indexing in INTERP_* opcodes

The hardware doesn't support it, so we just interpolate all array elements
and then use indirect indexing on the resulting vector.

Clearly, this is not very efficient. There is an argument to be had for
adding if/else, or perhaps even pulling the data out of LDS directly.
Both don't really seem worth the effort, considering that it seems nobody
actually uses this feature.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>