x64: Add native lowering for scalar fma (#4539)

Use `vfmadd213{ss,sd}` for these lowerings.

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -1847,7 +1847,12 @@ fn x64_get_operands<F: Fn(VReg) -> VReg>(inst: &Inst, collector: &mut OperandCol
             // Vfmadd uses and defs the dst reg, that is not the case with all
             // AVX's ops, if you're adding a new op, make sure to correctly define
             // register uses.
-            assert!(*op == AvxOpcode::Vfmadd213ps || *op == AvxOpcode::Vfmadd213pd);
+            assert!(
+                *op == AvxOpcode::Vfmadd213ss
+                    || *op == AvxOpcode::Vfmadd213sd
+                    || *op == AvxOpcode::Vfmadd213ps
+                    || *op == AvxOpcode::Vfmadd213pd
+            );
 
             collector.reg_use(src1.to_reg());
             collector.reg_reuse_def(dst.to_writable_reg(), 0);