riscv64: Improve ctz/clz/cls codegen (#5854)

* cranelift: Add extra runtests for `clz`/`ctz`

* riscv64: Restrict lowering rules for `ctz`/`clz`

* cranelift: Add `u64` isle helpers

* riscv64: Improve `ctz` codegen

* riscv64: Improve `clz` codegen

* riscv64: Improve `cls` codegen

* riscv64: Improve `clz.i128` codegen

Instead of checking if we have 64 zeros in the top half. Check
if it *is* 0, that way we avoid loading the `64` constant.

* riscv64: Improve `ctz.i128` codegen

Instead of checking if we have 64 zeros in the bottom half. Check
if it *is* 0, that way we avoid loading the `64` constant.

* riscv64: Use extended value in `lower_cls`

* riscv64: Use pattern matches on `bseti`

cranelift/codegen/src/isle_prelude.rs

@@ -38,6 +38,11 @@ macro_rules! isle_common_prelude_methods {
             x as u64
         }
 
+        #[inline]
+        fn u64_as_i32(&mut self, x: u64) -> i32 {
+            x as i32
+        }
+
         #[inline]
         fn i64_neg(&mut self, x: i64) -> i64 {
             x.wrapping_neg()