[codegen] Make scalar_to_vector's output type a lane of its input type;

cranelift/filetests/filetests/verifier/scalar-to-vector.clif

@@ -0,0 +1,10 @@
+test verifier
+set enable_simd=true
+target x86_64
+
+function %scalar_to_vector() {
+ebb0:
+    v0 = iconst.i32 42
+    v1 = scalar_to_vector.f32x4 v0 ; error: arg 0 (v0) has type i32, expected f32
+    return
+}