i965/vs: Add simple dead code elimination.

This is copied right from the fragment shader.  It is needed for real
register allocation to work correctly.

src/mesa/drivers/dri/i965/brw_vec4.cpp

@@ -127,4 +127,35 @@ vec4_visitor::virtual_grf_interferes(int a, int b)
    return start < end;
 }
 
+/**
+ * Must be called after calculate_live_intervales() to remove unused
+ * writes to registers -- register allocation will fail otherwise
+ * because something deffed but not used won't be considered to
+ * interfere with other regs.
+ */
+bool
+vec4_visitor::dead_code_eliminate()
+{
+   bool progress = false;
+   int pc = 0;
+
+   calculate_live_intervals();
+
+   foreach_list_safe(node, &this->instructions) {
+      vec4_instruction *inst = (vec4_instruction *)node;
+
+      if (inst->dst.file == GRF && this->virtual_grf_use[inst->dst.reg] <= pc) {
+	 inst->remove();
+	 progress = true;
+      }
+
+      pc++;
+   }
+
+   if (progress)
+      live_intervals_valid = false;
+
+   return progress;
+}
+
 } /* namespace brw */

src/mesa/drivers/dri/i965/brw_vec4.h

@@ -381,6 +381,7 @@ public:
    void reg_allocate();
    void move_grf_array_access_to_scratch();
    void calculate_live_intervals();
+   bool dead_code_eliminate();
    bool virtual_grf_interferes(int a, int b);
 
    vec4_instruction *emit(enum opcode opcode);

src/mesa/drivers/dri/i965/brw_vec4_emit.cpp

@@ -558,6 +558,12 @@ vec4_visitor::run()
     */
    move_grf_array_access_to_scratch();
 
+   bool progress;
+   do {
+      progress = false;
+      progress = dead_code_eliminate() || progress;
+   } while (progress);
+
    if (failed)
       return false;