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

lib/frontend/src/frontend.rs

@@ -591,7 +591,7 @@ impl<'a> FunctionBuilder<'a> {
             colocated: false,
         });
 
-        self.ins().uextend(types::I32, ch);
+        let ch = self.ins().uextend(types::I32, ch);
         self.ins().call(libc_memset, &[buffer, ch, len]);
     }