mesa: updated comment

(cherry picked from commit d9fa9e3290)

src/mesa/shader/slang/slang_codegen.c

@@ -1474,6 +1474,8 @@ _slang_simple_writemask(GLuint writemask, GLuint swizzle)
  * Convert the given swizzle into a writemask.  In some cases this
  * is trivial, in other cases, we'll need to also swizzle the right
  * hand side to put components in the right places.
+ * See comment above for more info.
+ * XXX this function could be simplified and should probably be renamed.
  * \param swizzle  the incoming swizzle
  * \param writemaskOut  returns the writemask
  * \param swizzleOut  swizzle to apply to the right-hand-side