Legalize some more i8/i16 intructions (#524)

* Legalize some more i8/i16 intructions

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

@@ -0,0 +1,19 @@
+test compile
+target x86_64
+
+; regex: V=v\d+
+
+function u0:0(i8, i8) -> i8 fast {
+ebb0(v0: i8, v1: i8):
+    v2 = icmp_imm sle v0, 0
+    ; check: $(e1=$V) = sextend.i32 v0
+    ; nextln: v2 = icmp_imm sle $e1, 0
+    v3 = bint.i8 v2
+    v4 = icmp eq v0, v1
+    ; check: $(e2=$V) = uextend.i32 v0
+    ; nextln: $(e3=$V) = uextend.i32 v1
+    ; nextln: v4 = icmp eq $e2, $e3
+    v5 = bint.i8 v4
+    v6 = iadd v3, v5
+    return v6
+}