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/machinst/lower.rs

@@ -13,8 +13,8 @@ use crate::ir::instructions::BranchInfo;
 use crate::ir::{
     types::{FFLAGS, IFLAGS},
     ArgumentPurpose, Block, Constant, ConstantData, DataFlowGraph, ExternalName, Function,
-    GlobalValueData, Inst, InstructionData, MemFlags, Opcode, Signature, SourceLoc, Type, Value,
-    ValueDef, ValueLabelAssignments, ValueLabelStart,
+    GlobalValue, GlobalValueData, Inst, InstructionData, MemFlags, Opcode, Signature, SourceLoc,
+    Type, Value, ValueDef, ValueLabelAssignments, ValueLabelStart,
 };
 use crate::machinst::{
     non_writable_value_regs, writable_value_regs, ABICallee, BlockIndex, BlockLoweringOrder,
@@ -93,6 +93,11 @@ pub trait LowerCtx {
     /// Get the symbol name, relocation distance estimate, and offset for a
     /// symbol_value instruction.
     fn symbol_value<'b>(&'b self, ir_inst: Inst) -> Option<(&'b ExternalName, RelocDistance, i64)>;
+    /// Likewise, but starting with a GlobalValue identifier.
+    fn symbol_value_data<'b>(
+        &'b self,
+        global_value: GlobalValue,
+    ) -> Option<(&'b ExternalName, RelocDistance, i64)>;
     /// Returns the memory flags of a given memory access.
     fn memflags(&self, ir_inst: Inst) -> Option<MemFlags>;
     /// Get the source location for a given instruction.
@@ -1070,19 +1075,26 @@ impl<'func, I: VCodeInst> LowerCtx for Lower<'func, I> {
     fn symbol_value<'b>(&'b self, ir_inst: Inst) -> Option<(&'b ExternalName, RelocDistance, i64)> {
         match &self.f.dfg[ir_inst] {
             &InstructionData::UnaryGlobalValue { global_value, .. } => {
-                let gvdata = &self.f.global_values[global_value];
-                match gvdata {
-                    &GlobalValueData::Symbol {
-                        ref name,
-                        ref offset,
-                        ..
-                    } => {
-                        let offset = offset.bits();
-                        let dist = gvdata.maybe_reloc_distance().unwrap();
-                        Some((name, dist, offset))
-                    }
-                    _ => None,
-                }
+                self.symbol_value_data(global_value)
+            }
+            _ => None,
+        }
+    }
+
+    fn symbol_value_data<'b>(
+        &'b self,
+        global_value: GlobalValue,
+    ) -> Option<(&'b ExternalName, RelocDistance, i64)> {
+        let gvdata = &self.f.global_values[global_value];
+        match gvdata {
+            &GlobalValueData::Symbol {
+                ref name,
+                ref offset,
+                ..
+            } => {
+                let offset = offset.bits();
+                let dist = gvdata.maybe_reloc_distance().unwrap();
+                Some((name, dist, offset))
             }
             _ => None,
         }