ISLE: Common accessors for some insn data fields (#3781)

Add accessors to prelude.isle to access data fields of
`func_addr` and `symbol_value` instructions.

These are based on similar versions I had added to the s390x
back-end, but are a bit more straightforward to use.

- func_ref_data: Extract SigRef, ExternalName, and RelocDistance
  fields given a FuncRef.

- symbol_value_data: Extract ExternalName, RelocDistance, and
  offset fields given a GlobalValue representing a Symbol.

- reloc_distance_near: Test for RelocDistance::Near.

The s390x back-end is changed to use these common versions.

Note that this exposed a bug in common isle code: This extractor:

(extractor (load_sym inst)
  (and inst
       (load _ (def_inst (symbol_value
                           (symbol_value_data _
                             (reloc_distance_near) offset)))
               (i64_from_offset
                 (memarg_symbol_offset_sum <offset _)))))

would raise an assertion in sema.rs due to a supposed cycle in
extractor definitions.  But there was no actual cycle, it was
simply that the extractor tree refers twice to the `insn_data`
extractor (once via the `load` and once via the `symbol_value`
extractor).  Fixed by checking for pre-existing definitions only
along one path in the tree, not across the whole tree.

cranelift/codegen/src/isa/s390x/lower.isle

@@ -1156,27 +1156,23 @@
 ;;;; Rules for `func_addr` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 ;; Load the address of a function, target reachable via PC-relative instruction.
-(rule (lower (and (func_addr _)
-                  (call_target_data name (reloc_distance_near))))
+(rule (lower (func_addr (func_ref_data _ name (reloc_distance_near))))
       (value_reg (load_addr (memarg_symbol name 0 (memflags_trusted)))))
 
 ;; Load the address of a function, general case.
-(rule (lower (and (func_addr _)
-                  (call_target_data name _)))
+(rule (lower (func_addr (func_ref_data _ name _)))
       (value_reg (load_ext_name_far name 0)))
 
 
 ;;;; Rules for `symbol_value` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 ;; Load the address of a symbol, target reachable via PC-relative instruction.
-(rule (lower (and (symbol_value _)
-                  (symbol_value_data name (reloc_distance_near)
-                                          (memarg_symbol_offset offset))))
+(rule (lower (symbol_value (symbol_value_data name (reloc_distance_near)
+                                                   (memarg_symbol_offset offset))))
       (value_reg (load_addr (memarg_symbol name offset (memflags_trusted)))))
 
 ;; Load the address of a symbol, general case.
-(rule (lower (and (symbol_value _)
-                  (symbol_value_data name _ offset)))
+(rule (lower (symbol_value (symbol_value_data name _ offset)))
       (value_reg (load_ext_name_far name offset)))