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-load-store-i8.cton

@@ -0,0 +1,31 @@
+test compile
+
+target x86_64
+
+function u0:0(i64, i8, i8) system_v {
+    ss0 = explicit_slot 0
+    ss1 = explicit_slot 1
+    ss2 = explicit_slot 1
+    ss3 = explicit_slot 1
+    ss4 = explicit_slot 1
+
+ebb0(v0: i64, v1: i8, v2: i8):
+    v3 = stack_addr.i64 ss1
+    store v1, v3
+    v4 = stack_addr.i64 ss2
+    store v2, v4
+    v5 = stack_addr.i64 ss3
+    v6 = stack_addr.i64 ss4
+    jump ebb1
+
+ebb1:
+    v7 = load.i8 v3
+    store v7, v5
+    v8 = load.i8 v4
+    store v8, v6
+    v9 = load.i8 v5
+    v10 = load.i8 v6
+    v11 = imul v9, v10
+    store v11, v0
+    return
+}