llvm-reduce: Improve delta pass flag handling

llvm-reduce: Improve delta pass flag handling

Verify all the requested passes exist before trying to run any.
For long reductions, it was really annoying for it to get halfway through
and then I come back later to an incomplete reduction.

Also add a new skip-delta-passes flag. Most of the time I want to opt out
of specific reductions, rather than run a select few.