Do LVN to flag regsiter mov

Reduce extra flag register mov instructions

Author: bcheng0127

visa/Passes/LVN.cpp

@@ -1041,11 +1041,22 @@ bool LVN::addValue(G4_INST *inst) {
 
   G4_Operand *dst = inst->getDst();
   if (!dst->getBase() || !dst->getBase()->isRegVar() ||
-      dst->getBase()->asRegVar()->getDeclare()->getRegFile() != G4_GRF ||
+      (dst->getBase()->asRegVar()->getDeclare()->getRegFile() != G4_GRF &&
+       dst->getBase()->asRegVar()->getDeclare()->getRegFile() != G4_FLAG) ||
       dst->getTopDcl()->getAddressed()) {
     return false;
   }
 
+  // Only handle mov flag, imm
+  if (dst->getBase()->asRegVar()->getDeclare()->getRegFile() == G4_FLAG) {
+    if (inst->opcode() != G4_mov) {
+      return false;
+    }
+    if (!inst->getSrc(0)->isImm()) {
+      return false;
+    }
+  }
+
   if (dst->getBase() && dst->getTopDcl()->isOutput()) {
     return false;
   }