Ensure that GenerateStoreFastDoubleElement returns stored value on all paths.

src/arm/stub-cache-arm.cc

Index: src/arm/stub-cache-arm.cc
===================================================================
--- src/arm/stub-cache-arm.cc	(revision 8772)
+++ src/arm/stub-cache-arm.cc	(working copy)
@@ -4399,11 +4399,18 @@
   } else {
     destination = FloatingPointHelper::kCoreRegisters;
   }
-  __ SmiUntag(value_reg, value_reg);
+
+  Register untagged_value = receiver_reg;
+  __ SmiUntag(untagged_value, value_reg);
   FloatingPointHelper::ConvertIntToDouble(
-      masm, value_reg, destination,
-      d0, mantissa_reg, exponent_reg,  // These are: double_dst, dst1, dst2.
-      scratch4, s2);  // These are: scratch2, single_scratch.
+      masm,
+      untagged_value,
+      destination,
+      d0,
+      mantissa_reg,
+      exponent_reg,
+      scratch4,
+      s2);
   if (destination == FloatingPointHelper::kVFPRegisters) {
     CpuFeatures::Scope scope(VFP3);
     __ vstr(d0, scratch, 0);