C++: Decrease largeVariable cut-off to 100k

This 10x lower cut-off has on at least one snapshot made it possible to
compute AST data flow where it was infeasible before.

Also fix an integer overflow that happened in practice on at least one
snapshot and prevented the cut-off from being applied.

cpp/ql/src/semmle/code/cpp/dataflow/internal/FlowVar.qll

@@ -620,7 +620,8 @@ module FlowVar_internal {
   private predicate largeVariable(Variable v, int liveBlocks, int defs) {
     liveBlocks = strictcount(SubBasicBlock sbb | variableLiveInSBB(sbb, v)) and
     defs = strictcount(SubBasicBlock sbb | exists(TBlockVar(sbb, v))) and
-    liveBlocks * defs > 1000000
+    // Convert to float to avoid int overflow (32-bit two's complement)
+    liveBlocks.(float) * defs.(float) > 100000.0
   }
 
   /**