Implemented the RoundInt64ToFloat32 TurboFan operator for x64, arm64, and mips64.

src/compiler/x64/code-generator-x64.cc

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index ccafb3cbed2e26f1821697b8360302982054dfbc..9911882bfc1f949352b02c4a90528c9969a42d6f 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -953,6 +953,13 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
         __ Cvtlsi2sd(i.OutputDoubleRegister(), i.InputOperand(0));
       }
       break;
+    case kSSEInt64ToFloat32:
+      if (instr->InputAt(0)->IsRegister()) {
+        __ Cvtqsi2ss(i.OutputDoubleRegister(), i.InputRegister(0));
+      } else {
+        __ Cvtqsi2ss(i.OutputDoubleRegister(), i.InputOperand(0));
+      }
+      break;
     case kSSEInt64ToFloat64:
       if (instr->InputAt(0)->IsRegister()) {
         __ Cvtqsi2sd(i.OutputDoubleRegister(), i.InputRegister(0));