C++: Use an 'EquivalenceRelation' instead of the 'shortestDistances' HOP in 'getInstruction'. This reduces the memory pressure when generating the CFG for Wireshark.

cpp/ql/lib/semmle/code/cpp/ir/implementation/aliased_ssa/IRBlock.qll

@@ -255,14 +255,27 @@ private module Cached {
   cached
   newtype TIRBlock = MkIRBlock(Instruction firstInstr) { startsBasicBlock(firstInstr) }
 
-  /** Holds if `i` is the `index`th instruction the block starting with `first`. */
-  private Instruction getInstructionFromFirst(Instruction first, int index) =
-    shortestDistances(startsBasicBlock/1, adjacentInBlock/2)(first, result, index)
+  /** Gets the index of `i` in its `IRBlock`. */
+  private int getMemberIndex(Instruction i) {
+    startsBasicBlock(i) and
+    result = 0
+    or
+    exists(Instruction iPrev |
+      adjacentInBlock(iPrev, i) and
+      result = getMemberIndex(iPrev) + 1
+    )
+  }
+
+  private module BlockAdjacency = QlBuiltins::EquivalenceRelation<Instruction, adjacentInBlock/2>;
 
   /** Holds if `i` is the `index`th instruction in `block`. */
   cached
   Instruction getInstruction(TIRBlock block, int index) {
-    result = getInstructionFromFirst(getFirstInstruction(block), index)
+    exists(Instruction first |
+      block = MkIRBlock(first) and
+      index = getMemberIndex(result) and
+      BlockAdjacency::getEquivalenceClass(first) = BlockAdjacency::getEquivalenceClass(result)
+    )
   }
 
   cached