Say kNumDalvikInstructions when appropriate.

In particular, use it instead of just saying 256, and similarly for
255. The number of opcodes will be changing soon.

Change-Id: Icc77120c2673968dddd6b4003f717245d46e4159
diff --git a/vm/compiler/CompilerIR.h b/vm/compiler/CompilerIR.h
index 82b97e5..c83c0ba 100644
--- a/vm/compiler/CompilerIR.h
+++ b/vm/compiler/CompilerIR.h
@@ -78,7 +78,7 @@
 } LIR;
 
 enum ExtendedMIROpcode {
-    kMirOpFirst = 256,
+    kMirOpFirst = kNumDalvikInstructions,
     kMirOpPhi = kMirOpFirst,
     kMirOpNullNRangeUpCheck,
     kMirOpNullNRangeDownCheck,