glsl: Add conditional-select IR.

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>