i965/vec4: Let DCE eliminate dead writes in other basic blocks.

We previously stopped searching for unread writes after encountering
control flow, but we can instead just search backwards until we hit
control flow.

instructions in affected programs:     22854 -> 22194 (-2.89%)

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

@@ -378,7 +378,6 @@ bool
 vec4_visitor::dead_code_eliminate()
 {
    bool progress = false;
-   bool seen_control_flow = false;
    int pc = -1;
 
    calculate_live_intervals();
@@ -388,8 +387,6 @@ vec4_visitor::dead_code_eliminate()
 
       pc++;
 
-      seen_control_flow = inst->is_control_flow() || seen_control_flow;
-
       bool inst_writes_flag = false;
       if (inst->dst.file != GRF) {
          if (inst->dst.is_null() && inst->writes_flag()) {
@@ -415,7 +412,7 @@ vec4_visitor::dead_code_eliminate()
                     progress;
       }
 
-      if (seen_control_flow || inst->predicate || inst->prev == NULL)
+      if (inst->predicate || inst->prev == NULL)
          continue;
 
       int dead_channels;
@@ -443,6 +440,9 @@ vec4_visitor::dead_code_eliminate()
            node = prev, prev = prev->prev) {
          vec4_instruction *scan_inst = (vec4_instruction  *)node;
 
+         if (scan_inst->is_control_flow())
+            break;
+
          if (inst_writes_flag) {
             if (scan_inst->dst.is_null() && scan_inst->writes_flag()) {
                scan_inst->remove();