Quick Compiler: pointer/boolean cleanup

More minor code cleanup - follow the Art convention of not treating
pointers as booleans in "for" loop tests.

Change-Id: I2fcd06efe6a51d1195c0900f7fa110fc01110001
diff --git a/src/compiler/codegen/method_codegen_driver.cc b/src/compiler/codegen/method_codegen_driver.cc
index 9f7f692..fe5d522 100644
--- a/src/compiler/codegen/method_codegen_driver.cc
+++ b/src/compiler/codegen/method_codegen_driver.cc
@@ -729,7 +729,7 @@
     GenExitSequence(cu);
   }
 
-  for (mir = bb->first_mir_insn; mir; mir = mir->next) {
+  for (mir = bb->first_mir_insn; mir != NULL; mir = mir->next) {
     ResetRegPool(cu);
     if (cu->disable_opt & (1 << kTrackLiveTemps)) {
       ClobberAllRegs(cu);