Port icmp to ISLE (AArch64) (#4898)

* Port `icmp` to ISLE (AArch64)

Ported the existing implementation of `icmp` (and, by extension, the
`lower_icmp` function) to ISLE for AArch64.

Copyright (c) 2022 Arm Limited

* Allow 'producer chains', eliminating `Nop0`s

Copyright (c) 2022 Arm Limited

cranelift/codegen/src/isa/x64/inst.isle

@@ -1362,12 +1362,6 @@
 (decl intcc_without_eq (IntCC) IntCC)
 (extern constructor intcc_without_eq intcc_without_eq)
 
-;; This is a direct import of `IntCC::unsigned`.
-;; Get the corresponding IntCC with the signed component removed.
-;; For conditions without a signed component, this is a no-op.
-(decl intcc_unsigned (IntCC) IntCC)
-(extern constructor intcc_unsigned intcc_unsigned)
-
 ;;;; Helpers for Getting Particular Physical Registers ;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;; These should only be used for legalization purposes, when we can't otherwise

cranelift/codegen/src/isa/x64/lower/isle.rs

@@ -596,11 +596,6 @@ impl Context for IsleContext<'_, '_, MInst, Flags, IsaFlags, 6> {
         x.without_equal()
     }
 
-    #[inline]
-    fn intcc_unsigned(&mut self, x: &IntCC) -> IntCC {
-        x.unsigned()
-    }
-
     #[inline]
     fn intcc_to_cc(&mut self, intcc: &IntCC) -> CC {
         CC::from_intcc(*intcc)