Clobber non-live temporaries on some paths involving a possible slow-path allocation.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 94baf54c53b4848bfb74da1f58d5582143948ba4..d2b7f8ab0f588da1c93fb3a54c0cc124d8d47724 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -1864,6 +1864,9 @@ void StoreInstanceFieldInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
       locs()->live_registers()->Add(locs()->in(1));
     }
 
+    // Manually preserve temp which could be clobbered on the slow path.
+    locs()->live_registers()->Add(locs()->temp(0));
+
     {
       __ Bind(&store_double);
       EnsureMutableBox(compiler,