[turbofan] Add backend support for float32.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index b2a97cc64208572b297a65ea73a07f52dc61f0f9..f25c7a974ae4f426120b29d3b2d2788ff26abb4d 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -200,6 +200,11 @@ struct XMMRegister {
     return kMaxNumAllocatableRegisters;
   }
 
+  // TODO(turbofan): Proper support for float32.
+  static int NumAllocatableRegistersForTurbofan() {
+    return NumAllocatableRegisters();
+  }
+
   static int ToAllocationIndex(XMMRegister reg) {
     DCHECK(reg.code() != 0);
     return reg.code() - 1;