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-imul-i8.cton

@@ -0,0 +1,11 @@
+test compile
+
+target x86_64
+
+function u0:0(i64, i8, i8) system_v {
+
+ebb0(v0: i64, v1: i8, v2: i8):
+    v11 = imul v1, v2
+    store v11, v0
+    return
+}