Cranelift: Rewrite or(and(x, y), not(y)) => or(x, not(y)) again (#5684)

This rewrite was introduced in #5676 and then reverted in #5682 due to a footgun
where we accidentally weren't actually checking the `y == !z` precondition. This
commit fixes the precondition check. It also fixes the arithmetic to be
correctly masked to the value type's width.

This reverts commit 268f6bfc1d.

cranelift/codegen/src/isle_prelude.rs

@@ -109,6 +109,11 @@ macro_rules! isle_common_prelude_methods {
             !x
         }
 
+        #[inline]
+        fn u64_eq(&mut self, x: u64, y: u64) -> bool {
+            x == y
+        }
+
         #[inline]
         fn u64_is_zero(&mut self, value: u64) -> bool {
             0 == value