glsl: Flip around "if" statements with empty "then" blocks.

This cleans up some funny-looking code in some unigine shaders I was
looking at.  Also slightly helps on planeshift and a few shaders in an
upcoming Valve release.

total instructions in shared programs: 1653715 -> 1653587 (-0.01%)
instructions in affected programs:     16550 -> 16422 (-0.77%)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/glsl/opt_if_simplification.cpp

@@ -25,7 +25,8 @@
  * \file opt_if_simplification.cpp
  *
  * Moves constant branches of if statements out to the surrounding
- * instruction stream.
+ * instruction stream, and inverts if conditionals to avoid empty
+ * "then" blocks.
  */
 
 #include "ir.h"
@@ -101,6 +102,30 @@ ir_if_simplification_visitor::visit_leave(ir_if *ir)
       }
       ir->remove();
       this->made_progress = true;
+      return visit_continue;
+   }
+
+   /* Turn:
+    *
+    *     if (cond) {
+    *     } else {
+    *         do_work();
+    *     }
+    *
+    * into :
+    *
+    *     if (!cond)
+    *         do_work();
+    *
+    * which avoids control flow for "else" (which is usually more
+    * expensive than normal operations), and the "not" can usually be
+    * folded into the generation of "cond" anyway.
+    */
+   if (ir->then_instructions.is_empty()) {
+      ir->condition = new(ralloc_parent(ir->condition))
+	 ir_expression(ir_unop_logic_not, ir->condition);
+      ir->else_instructions.move_nodes_to(&ir->then_instructions);
+      this->made_progress = true;
    }
 
    return visit_continue;