nir: Add nir_texop_samples_identical opcode

This is the NIR analog to GLSL IR ir_samples_identical.

v2: Don't add the second nir_tex_src_ms_index parameter.  Suggested by
Ken and Jason.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
6 files changed