glsl2: Add ir_assignment::write_mask and associated methods

Replace swizzles on the LHS with additional swizzles on the RHS and a
write mask in the assignment instruction.  As part of this add
ir_assignment::set_lhs.  Ideally we'd make ir_assignment::lhs private
to prevent erroneous writes, but that would require a lot of code
butchery at this point.

Add ir_assignment constructor that takes an explicit write mask.  This
is required for ir_assignment::clone, but it can also be used in other
places.  Without this, ir_assignment clones lose their write masks,
and incorrect IR is generated in optimization passes.

Add ir_assignment::whole_variable_written method.  This method gets
the variable on the LHS if the whole variable is written or NULL
otherwise.  This is different from
ir->lhs->whole_variable_referenced() because the latter has no
knowledge of the write mask stored in the ir_assignment.

Gut all code from ir_to_mesa that handled swizzles on the LHS of
assignments.  There is probably some other refactoring that could be
done here, but that can be left for another day.

src/glsl/ir_clone.cpp

@@ -242,7 +242,8 @@ ir_assignment::clone(void *mem_ctx, struct hash_table *ht) const
 
    return new(mem_ctx) ir_assignment(this->lhs->clone(mem_ctx, ht),
 				     this->rhs->clone(mem_ctx, ht),
-				     new_condition);
+				     new_condition,
+				     this->write_mask);
 }
 
 ir_function *