[turbofan] Implemented the optional Float32RoundDown operator.

src/compiler/arm64/code-generator-arm64.cc

Index: src/compiler/arm64/code-generator-arm64.cc
diff --git a/src/compiler/arm64/code-generator-arm64.cc b/src/compiler/arm64/code-generator-arm64.cc
index 01bb31a87553b5886398584723fdb1b114da1b78..bd6e91eec0385cae53013b3a6195ac1272548c06 100644
--- a/src/compiler/arm64/code-generator-arm64.cc
+++ b/src/compiler/arm64/code-generator-arm64.cc
@@ -622,6 +622,9 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
       __ Bind(ool->exit());
       break;
     }
+    case kArm64Float32RoundDown:
+      __ Frintm(i.OutputFloat32Register(), i.InputFloat32Register(0));
+      break;
     case kArm64Float64RoundDown:
       __ Frintm(i.OutputDoubleRegister(), i.InputDoubleRegister(0));
       break;