glsl: Comment that expression flattening is used for matrix operations.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/glsl/ir_expression_flattening.cpp

@@ -27,7 +27,10 @@
  * Takes the leaves of expression trees and makes them dereferences of
  * assignments of the leaves to temporaries, according to a predicate.
  *
- * This is used for automatic function inlining, where we want to take
+ * This is used for breaking down matrix operations, where it's easier to
+ * create a temporary and work on each of its vector components individually.
+ *
+ * It is also used for automatic function inlining, where we want to take
  * an expression containing a call and move the call out to its own
  * assignment so that we can inline it at the appropriate place in the
  * instruction stream.