[DwarfEHPrepare] Don't prune unreachable resumes at optnone

Disable pruning of unreachable resumes in the DwarfEHPrepare pass
at optnone. While I expect the pruning itself to be essentially free,
this does require a dominator tree calculation, that is not used for
anything else. Saving this DT construction makes for a 0.4% O0
compile-time improvement.

Differential Revision: https://reviews.llvm.org/D80400