[GLOP] add presolve parameter

ortools/glop/parameters.proto

@@ -22,7 +22,7 @@ syntax = "proto2";
 
 package operations_research.glop;
 
-// next id = 67
+// next id = 68
 message GlopParameters {
 
   // Supported algorithms for scaling:
@@ -469,4 +469,7 @@ message GlopParameters {
   // SetStartingVariableValuesForNextSolve().
   optional bool push_to_vertex = 65 [default = true];
 
+  // If presolve runs, include the pass that detects implied free variables.
+  optional bool use_implied_free_preprocessor = 67 [default = true];
+
 }

ortools/glop/preprocessor.cc

@@ -1381,6 +1381,7 @@ struct ColWithDegree {
 bool ImpliedFreePreprocessor::Run(LinearProgram* lp) {
   SCOPED_INSTRUCTION_COUNT(time_limit_);
   RETURN_VALUE_IF_NULL(lp, false);
+  if (!parameters_.use_implied_free_preprocessor()) return false;
   const RowIndex num_rows = lp->num_constraints();
   const ColIndex num_cols = lp->num_variables();