Legalize several i8 insts (#380)

* Legalize several i8 insts

* X86: implement regmove.{i8,i16}

* Legalize bnot

* Remove comments

* Nicer type param binding in legalize.py

* Legalize sdiv_imm.i8

* Hopefully fix mypy error

* Add missing trailing newlines

* Fix tests

cranelift/filetests/isa/x86/legalize-iconst-i8.cton

@@ -0,0 +1,18 @@
+test compile
+
+target x86_64
+
+function u0:0(i64) system_v {
+    ss0 = explicit_slot 0
+
+ebb0(v0: i64):
+    jump ebb1
+
+ebb1:
+; _0 = const 42u8
+    v1 = iconst.i8 42
+    store v1, v0
+; 
+; return
+    return
+}