C#: Speedup bestImplementation

Avoids an expensive anti-join:

```
[2023-08-29 15:25:48] Evaluated non-recursive predicate _FileSystem#df18ed9a::Make#File#1a556f64::Input#::Container::toString#0#dispred#bf_Method#621e9e2e::__#antijoin_rhs@96d08bc8 in 272332ms (size: 1841891).
Evaluated relational algebra for predicate _FileSystem#df18ed9a::Make#File#1a556f64::Input#::Container::toString#0#dispred#bf_Method#621e9e2e::__#antijoin_rhs@96d08bc8 with tuple counts:
         4632443     ~2%    {3} r1 = JOIN _cil_instruction_3#antijoin_rhs_cil_method_implementation#shared WITH cil_method_implementation ON FIRST 1 OUTPUT Rhs.1, Lhs.0, Lhs.1

        71945701     ~3%    {3} r2 = JOIN r1 WITH cil_method_implementation_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2
        71945701  ~1329%    {3} r3 = JOIN r2 WITH Method#621e9e2e::MethodImplementation::getNumberOfInstructions#0#dispred#ff ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Rhs.1
         5016836     ~4%    {4} r4 = JOIN r3 WITH Method#621e9e2e::MethodImplementation::getNumberOfInstructions#0#dispred#ff ON FIRST 1 OUTPUT Lhs.0, Lhs.1, Lhs.2, Rhs.1
                            {4} r5 = SELECT r4 ON In.3 < In.2
           65637     ~3%    {2} r6 = SCAN r5 OUTPUT In.0, In.1

        71945701     ~0%    {3} r7 = JOIN r1 WITH cil_method_implementation_10#join_rhs ON FIRST 1 OUTPUT Lhs.2, Lhs.1, Rhs.1
        71945701     ~1%    {4} r8 = JOIN r7 WITH assemblies ON FIRST 1 OUTPUT Lhs.2, Lhs.1, Lhs.0, Rhs.1
        71945701     ~0%    {5} r9 = JOIN r8 WITH cil_method_implementation ON FIRST 1 OUTPUT Rhs.2, Lhs.1, Lhs.2, Lhs.0, Lhs.3
        71945701     ~0%    {5} r10 = JOIN r9 WITH assemblies ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2, Lhs.3, Lhs.4
        71945701     ~0%    {5} r11 = JOIN r10 WITH FileSystem#df18ed9a::Make#File#1a556f64::Input#::Container::toString#0#dispred#bf ON FIRST 1 OUTPUT Lhs.4, Lhs.1, Lhs.2, Lhs.3, Rhs.1
        71945701     ~2%    {5} r12 = JOIN r11 WITH FileSystem#df18ed9a::Make#File#1a556f64::Input#::Container::toString#0#dispred#bf ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.3, Lhs.4, Rhs.1
                            {5} r13 = SELECT r12 ON In.4 > In.3
        33509342     ~0%    {3} r14 = SCAN r13 OUTPUT In.0, In.1, In.2
        33509342     ~0%    {4} r15 = JOIN r14 WITH Method#621e9e2e::MethodImplementation::getNumberOfInstructions#0#dispred#ff ON FIRST 1 OUTPUT Lhs.2, Rhs.1, Lhs.0, Lhs.1
        33051362  ~1670%    {2} r16 = JOIN r15 WITH Method#621e9e2e::MethodImplementation::getNumberOfInstructions#0#dispred#ff ON FIRST 2 OUTPUT Lhs.2, Lhs.3

        33116999  ~1646%    {2} r17 = r6 UNION r16
                            return r17
```

Author: hvitved

csharp/ql/lib/semmle/code/cil/Stubs.qll

@@ -32,12 +32,12 @@ private module Cached {
     exists(Assembly asm |
       asm = mi.getLocation() and
       (assemblyIsStubImpl(asm) implies asm.getFile().extractedQlTest()) and
-      not exists(MethodImplementation better | mi.getMethod() = better.getMethod() |
-        mi.getNumberOfInstructions() < better.getNumberOfInstructions()
-        or
-        mi.getNumberOfInstructions() = better.getNumberOfInstructions() and
-        asm.getFile().toString() > better.getLocation().getFile().toString()
-      ) and
+      mi =
+        max(MethodImplementation impl |
+          mi.getMethod() = impl.getMethod()
+        |
+          impl order by impl.getNumberOfInstructions(), impl.getLocation().getFile().toString() desc
+        ) and
       exists(mi.getAnInstruction())
     )
   }