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/prelude.isle

@@ -57,7 +57,7 @@
 ;; Extractor that pulls apart an Offset32 into a u32 with the raw
 ;; signed-32-bit twos-complement bits.
 (decl offset32 (u32) Offset32)
-(extern extractor offset32 offset32)
+(extern extractor infallible offset32 offset32)
 
 ;; Pure/fallible constructor that tests if one u32 is less than or
 ;; equal to another.