Ensure that BoxInt32 input is correctly preserved on the slow-path.

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 2c1d14ca9ab5aba5dea805f715e8d1240bf182b3..0468d643e1367e4f0cd8da4402f298b430c1cf3f 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -1681,6 +1681,12 @@ void StoreInstanceFieldInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
     Register temp2 = locs()->temp(1).reg();
     FpuRegister fpu_temp = locs()->temp(2).fpu_reg();
 
+    if (ShouldEmitStoreBarrier()) {
+      // Value input is a writable register and should be manually preserved
+      // across allocation slow-path.
+      locs()->live_registers()->Add(locs()->in(1), kTagged);
+    }
+
     Label store_pointer;
     Label store_double;
     Label store_float32x4;