ISLE rule cleanups (#5389)

* cranelift-codegen: Use ISLE matching, not same_value

The `same_value` function just wrapped an equality test into an external
constructor, but we can do that with ISLE's equality constraints
instead.

* riscv64: Remove custom condition-code tests

The `lower_icmp` term exists solely to decide whether to sign-extend or
zero-extend the comparison operands, based on whether the condition code
requires a signed comparison. It additionally tested whether the
condition code was == or !=, but produced the same result as for other
unsigned comparisons.

We already have `signed_cond_code` in the ISLE prelude, which classifies
the total-ordering condition codes according to whether they're signed.
It also lumps == and != in the "unsigned" camp, as desired.

So this commit uses the existing method from the prelude instead of
riscv64-local definitions.

Because this version has no constraints on the left-hand side of the
rule in the unsigned case, ISLE generates Rust that always returns
`Some`. That shows that the current use of `unwrap` is justified, at the
only Rust-side call-site of `constructor_lower_icmp`, which is in
cranelift/codegen/src/isa/riscv64/lower/isle.rs.

* ISLE prelude: make offset32 infallible

This extractor always returns `Some`, so it doesn't need to be fallible.

cranelift/codegen/src/machinst/isle.rs

@@ -46,15 +46,6 @@ macro_rules! isle_lower_prelude_methods {
     () => {
         isle_common_prelude_methods!();
 
-        #[inline]
-        fn same_value(&mut self, a: Value, b: Value) -> Option<Value> {
-            if a == b {
-                Some(a)
-            } else {
-                None
-            }
-        }
-
         #[inline]
         fn value_type(&mut self, val: Value) -> Type {
             self.lower_ctx.dfg().value_type(val)