Make dynflag argument for withTiming pure.

19 times out of 20 we already have dynflags in scope.

We could just always use `return dflags`. But this is in fact not free.
When looking at some STG code I noticed that we always allocate a
closure for this expression in the heap. Clearly a waste in these cases.

For the other cases we can either just modify the callsite to
get dynflags or use the _D variants of withTiming I added which
will use getDynFlags under the hood.