Fix sign extension

Co-authored-by: Max Graey <maxgraey@gmail.com>

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -52,8 +52,8 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             let value = ctx.get_constant(insn).unwrap();
             // Sign extend constant if necessary
             let value = match ty.unwrap() {
-                I8 => (((value as i64) << 8) >> 8) as u64,
-                I16 => (((value as i64) << 16) >> 16) as u64,
+                I8 => (((value as i64) << 56) >> 56) as u64,
+                I16 => (((value as i64) << 48) >> 48) as u64,
                 I32 => (((value as i64) << 32) >> 32) as u64,
                 I64 | R64 => value,
                 ty if ty.is_bool() => value,