intel/fs: Refactor ALU source and destination handling to a separate function

intel/fs: Refactor ALU source and destination handling to a separate function

Other places will need to do this soon to properly handle source
swizzles.  The patch looks a little odd, but the change is pretty
straight forward.  All of the swizzle and mask handling is moved out,
but the code for handling move instructions and vecN instructions
remains in nir_emit_alu.

I'm not terribly pleased with the "need_dest" parameter, but
get_nir_dest is (somewhat surprisingly) destructive.  I am open to
suggestions of alternatives.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>