Simplify LogicMTask/ExecMTask IDs (#4990)

There is no strong need to re-map LogicMTask IDs and it just adds extra
processing. Instead we just allocate a separate set of ExecMTask IDs as
they are created, which can also be used as the unique profiling ID as
well. The only effect on the output of this is the change in mtask IDs
emitted, which was fairly arbitrary to begin with.

src/V3LifePost.cpp

@@ -28,8 +28,8 @@
 
 #include "V3LifePost.h"
 
+#include "V3ExecGraph.h"
 #include "V3GraphPathChecker.h"
-#include "V3PartitionGraph.h"
 #include "V3Stats.h"
 
 #include <memory>  // for std::unique_ptr -> auto_ptr or unique_ptr