gallium: add PIPE_SHADER_CAP_SUPPORTED_IRS

This cap indicates the supported representations of programs. It should
be a mask of pipe_shader_ir bits. It will allow to enable
ARB_compute_shader if the underlying driver supports TGSI.

Changes from v2:
 - improve description of PIPE_SHADER_CAP_SUPPORTED_IRS

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
12 files changed