Remove IFLAGS/FFLAGS types (#5406)

All instructions using the CPU flags types (IFLAGS/FFLAGS) were already
removed.  This patch completes the cleanup by removing all remaining
instructions that define values of CPU flags types, as well as the
types themselves.

Specifically, the following features are removed:
- The IFLAGS and FFLAGS types and the SpecialType category.
- Special handling of IFLAGS and FFLAGS in machinst/isle.rs and
  machinst/lower.rs.
- The ifcmp, ifcmp_imm, ffcmp, iadd_ifcin, iadd_ifcout, iadd_ifcarry,
  isub_ifbin, isub_ifbout, and isub_ifborrow instructions.
- The writes_cpu_flags instruction property.
- The flags verifier pass.
- Flags handling in the interpreter.

All of these features are currently unused; no functional change
intended by this patch.

This addresses https://github.com/bytecodealliance/wasmtime/issues/3249.

cranelift/codegen/src/isa/aarch64/lower.isle

@@ -2332,36 +2332,6 @@
             (lower_msb Reg (lsr_imm $I64 lower_msb (imm_shift_from_u8 63))))
         (add_shift $I64 lower_msb upper_msb (lshl_from_u64 $I64 1))))
 
-;;; Rules for `iadd_ifcout` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-
-;; This is a two-output instruction that is needed for the
-;; legalizer's explicit heap-check sequence, among possible other
-;; uses. Its second output is a flags output only ever meant to
-;; check for overflow using the
-;; `backend.unsigned_add_overflow_condition()` condition.
-;;
-;; Note that the CLIF validation will ensure that no flag-setting
-;; operation comes between this IaddIfcout and its use (e.g., a
-;; Trapif). Thus, we can rely on implicit communication through the
-;; processor flags rather than explicitly generating flags into a
-;; register. We simply use the variant of the add instruction that
-;; sets flags (`adds`) here.
-;;
-;; Note that the second output (the flags) need not be generated,
-;; because flags are never materialized into a register; the only
-;; instructions that can use a value of type `iflags` or `fflags`
-;; will look directly for the flags-producing instruction (which can
-;; always be found, by construction) and merge it.
-;;
-;; Now handle the iadd as above, except use an AddS opcode that sets
-;; flags.
-
-(rule (lower (has_type (ty_int ty)
-                       (iadd_ifcout a b)))
-      (output_pair
-       (add_with_flags ty a b)
-       (invalid_reg)))
-
 ;;; Rules for `iadd_cout` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 ;; For values smaller than a register, we do a normal `add` with both arguments