C++/C#: Cache Instruction.getResultIRType()

Most of the predicates on `Instruction` are thin wrappers around cached predicates in the `IRConstruction` or `SSAConstruction` modules. However, `getResultIRType()` has to join `Construction::getInstructionResultType()` with `LanguageType::getIRType()`. `getResultIRType()` is called frequently both within the IR code and by IR consumers, and that's a big join to have to repeat in multiple stages.

I looked at most of the other predicates in `Instruction.qll`, and didn't see any other predicates that met all of the criteria of "large, commonly called, and not already inline".

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

@@ -218,6 +218,7 @@ class Instruction extends Construction::TStageInstruction {
    * Gets the type of the result produced by this instruction. If the instruction does not produce
    * a result, its result type will be `IRVoidType`.
    */
+  cached
   final IRType getResultIRType() { result = getResultLanguageType().getIRType() }
 
   /**

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

@@ -218,6 +218,7 @@ class Instruction extends Construction::TStageInstruction {
    * Gets the type of the result produced by this instruction. If the instruction does not produce
    * a result, its result type will be `IRVoidType`.
    */
+  cached
   final IRType getResultIRType() { result = getResultLanguageType().getIRType() }
 
   /**

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

@@ -218,6 +218,7 @@ class Instruction extends Construction::TStageInstruction {
    * Gets the type of the result produced by this instruction. If the instruction does not produce
    * a result, its result type will be `IRVoidType`.
    */
+  cached
   final IRType getResultIRType() { result = getResultLanguageType().getIRType() }
 
   /**