Legalize some more i8/i16 intructions (#524)

* Legalize some more i8/i16 intructions

cranelift/filetests/isa/x86/legalize-urem-i8.clif

@@ -0,0 +1,15 @@
+test compile
+target x86_64
+
+; regex: V=v\d+
+
+function u0:0(i8, i8) -> i8 fast {
+ebb0(v0: i8, v1: i8):
+    v2 = urem v0, v1
+    ; check: $(a=$V) = uextend.i32 v0
+    ; nextln: $(b=$V) = uextend.i32 v1
+    ; nextln: $(c=$V) = iconst.i32 0
+    ; nextln: $(V), $(r=$V) = x86_udivmodx $a, $c, $b
+    ; nextln: v2 = ireduce.i8 $r
+    return v2
+}