Cranelift: refactoring of unwind info (#2289)

* factor common code

* move fde/unwind emit to more abstract level

* code_len -> function_size

* speedup block scanning

* better function_size calciulation

* Rename UnwindCode enums

cranelift/codegen/src/isa/x86/abi.rs

@@ -993,7 +993,7 @@ fn insert_common_epilogues(
         pos.goto_last_inst(block);
         if let Some(inst) = pos.current_inst() {
             if pos.func.dfg[inst].opcode().is_return() {
-                insert_common_epilogue(inst, stack_size, pos, reg_type, csrs, sp_arg_index);
+                insert_common_epilogue(inst, block, stack_size, pos, reg_type, csrs, sp_arg_index);
             }
         }
     }
@@ -1003,6 +1003,7 @@ fn insert_common_epilogues(
 /// This is used by common calling conventions such as System V.
 fn insert_common_epilogue(
     inst: ir::Inst,
+    block: ir::Block,
     stack_size: i64,
     pos: &mut EncCursor,
     reg_type: ir::types::Type,
@@ -1062,12 +1063,13 @@ fn insert_common_epilogue(
         assert!(csrs.iter(FPR).len() == 0);
     }
 
-    pos.func.epilogues_start.push(
+    pos.func.epilogues_start.push((
         first_fpr_load
             .or(sp_adjust_inst)
             .or(first_csr_pop_inst)
             .unwrap_or(fp_pop_inst),
-    );
+        block,
+    ));
 }
 
 #[cfg(feature = "unwind")]