[turbofan] Implemented the optional Float32RoundDown operator.

src/compiler/ia32/code-generator-ia32.cc

Index: src/compiler/ia32/code-generator-ia32.cc
diff --git a/src/compiler/ia32/code-generator-ia32.cc b/src/compiler/ia32/code-generator-ia32.cc
index f3626934dede6c7cd72b6a28db3c17d517305f51..900376bb403ca5b47c30972a8831610653a65e8b 100644
--- a/src/compiler/ia32/code-generator-ia32.cc
+++ b/src/compiler/ia32/code-generator-ia32.cc
@@ -636,6 +636,13 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
       __ xorps(i.OutputDoubleRegister(), kScratchDoubleReg);
       break;
     }
+    case kSSEFloat32Round: {
+      CpuFeatureScope sse_scope(masm(), SSE4_1);
+      RoundingMode const mode =
+          static_cast<RoundingMode>(MiscField::decode(instr->opcode()));
+      __ roundss(i.OutputDoubleRegister(), i.InputDoubleRegister(0), mode);
+      break;
+    }
     case kSSEFloat64Cmp:
       __ ucomisd(i.InputDoubleRegister(0), i.InputOperand(1));
       break;