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/prelude.isle

@@ -117,6 +117,7 @@
 (type UnwindInst (primitive UnwindInst))
 (type ExternalName (primitive ExternalName))
 (type BoxExternalName (primitive BoxExternalName))
+(type RelocDistance (primitive RelocDistance))
 
 ;;;; Primitive Type Conversions ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
@@ -380,3 +381,22 @@
 
 (decl avoid_div_traps () Type)
 (extern extractor avoid_div_traps avoid_div_traps)
+
+;;;; Helpers for accessing instruction data ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Accessor for `FuncRef`.
+
+(decl func_ref_data (SigRef ExternalName RelocDistance) FuncRef)
+(extern extractor infallible func_ref_data func_ref_data)
+
+;; Accessor for `GobalValue`.
+
+(decl symbol_value_data (ExternalName RelocDistance i64) GlobalValue)
+(extern extractor symbol_value_data symbol_value_data)
+
+;; Accessor for `RelocDistance`.
+
+(decl reloc_distance_near () RelocDistance)
+(extern extractor reloc_distance_near reloc_distance_near)
+
+