Cranelift: Generalize (x << k) >> k optimization (#5746)

* Generalize unsigned `(x << k) >> k` optimization

Split the existing rule into three parts:
- A dual of the rule for `(x >> k) << k` that is only valid for unsigned
  shifts.
- Known-bits analysis for `(band (uextend x) k)`.
- A new rule for converting `sextend` to `uextend` if the sign-extended
  bits are masked out anyway.

The first two together cover the existing rule.

* Generalize signed `(x << k) >> k` optimization

* Review comments

* Generalize sign-extending shifts further

The shifts can be eliminated even if the shift amount isn't exactly
equal to the difference in bit-widths between the narrow and wide types.

* Add filetests

cranelift/codegen/src/isle_prelude.rs

@@ -129,6 +129,16 @@ macro_rules! isle_common_prelude_methods {
             x == y
         }
 
+        #[inline]
+        fn u64_le(&mut self, x: u64, y: u64) -> bool {
+            x <= y
+        }
+
+        #[inline]
+        fn u64_lt(&mut self, x: u64, y: u64) -> bool {
+            x < y
+        }
+
         #[inline]
         fn u64_is_zero(&mut self, value: u64) -> bool {
             0 == value