Fix leaf floating point runtime calls on ARM (work still needed in simulator).

runtime/vm/simulator_arm.cc

Index: runtime/vm/simulator_arm.cc
===================================================================
--- runtime/vm/simulator_arm.cc	(revision 23505)
+++ runtime/vm/simulator_arm.cc	(working copy)
@@ -1494,12 +1494,13 @@
         set_register(R3, icount_);
         set_register(IP, icount_);
         set_register(LR, icount_);
-        float zap_fvalue = static_cast<float>(icount_);
-        for (int i = S0; i <= S15; i++) {
-          set_sregister(static_cast<SRegister>(i), zap_fvalue);
+        double zap_dvalue = static_cast<double>(icount_);
+        for (int i = D0; i <= D7; i++) {
+          set_dregister(static_cast<DRegister>(i), zap_dvalue);
         }
+        // The above loop also zaps overlapping registers S0-S15.
+        // Registers D8-D15 (overlapping with S16-S31) are preserved.
 #ifdef VFPv3_D32
-        double zap_dvalue = static_cast<double>(icount_);
         for (int i = D16; i <= D31; i++) {
           set_dregister(static_cast<DRegister>(i), zap_dvalue);
         }