glsl: Add conditional-select IR.

It's a ?: that operates per-component on vectors. Will be used in
upcoming lowering pass for ldexp and the implementation of frexp.

 csel(selector, a, b):
   per-component result = selector ? a : b

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/program/ir_to_mesa.cpp

@@ -1497,6 +1497,7 @@ ir_to_mesa_visitor::visit(ir_expression *ir)
    case ir_triop_bitfield_extract:
    case ir_triop_vector_insert:
    case ir_quadop_bitfield_insert:
+   case ir_triop_csel:
       assert(!"not supported");
       break;