Fix ARM debug build by moving an assert.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
===================================================================
--- src/arm/lithium-codegen-arm.cc	(revision 6261)
+++ src/arm/lithium-codegen-arm.cc	(working copy)
@@ -1638,6 +1638,7 @@
 
 
 void LCodeGen::DoLoadKeyedFastElement(LLoadKeyedFastElement* instr) {
+  ASSERT(instr->representation().IsTagged());
   Register elements = ToRegister(instr->elements());
   Register key = EmitLoadRegister(instr->key(), scratch0());
   Register result = ToRegister(instr->result());
@@ -1649,7 +1650,6 @@
   __ ldr(result, FieldMemOperand(scratch, FixedArray::kHeaderSize));
 
   // Check for the hole value.
-  ASSERT(r.IsTagged());
   __ LoadRoot(scratch, Heap::kTheHoleValueRootIndex);
   __ cmp(result, scratch);
   DeoptimizeIf(eq, instr->environment());