Add encodings for i8 and i16 copy, spill, fill, ireduce.i8.i16 (#534)

* Add encodings for i8 and i16 copy, spill, fill, ireduce.i8.i16

Also adds legalization for srem, irsub_imm, {u,s}extend.i16.i8

Fixes #477 cc #466

* Legalize popcnt, clz and ctz for i8 and i16

* Fix bug in call_memset

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

@@ -0,0 +1,9 @@
+test compile
+target x86_64
+
+function u0:0(i8) -> i8 fast {
+ebb0(v0: i8):
+    v1 = popcnt v0
+    ; check-not: sextend.i32 v0
+    return v1
+}