C++: Use the cached getInstructionCount

The object-oriented `IRBlock` interface was recomputing instruction
counts instead of using the cached count that had already been computed.

Author: jbj

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

@@ -60,7 +60,7 @@ class IRBlockBase extends TIRBlock {
   }
 
   final int getInstructionCount() {
-    result = strictcount(getInstruction(_))
+    result = getInstructionCount(this)
   }
 
   final FunctionIR getEnclosingFunctionIR() {

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/IRBlock.qll

@@ -60,7 +60,7 @@ class IRBlockBase extends TIRBlock {
   }
 
   final int getInstructionCount() {
-    result = strictcount(getInstruction(_))
+    result = getInstructionCount(this)
   }
 
   final FunctionIR getEnclosingFunctionIR() {

cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/IRBlock.qll

@@ -60,7 +60,7 @@ class IRBlockBase extends TIRBlock {
   }
 
   final int getInstructionCount() {
-    result = strictcount(getInstruction(_))
+    result = getInstructionCount(this)
   }
 
   final FunctionIR getEnclosingFunctionIR() {