[PM] Simplify how we parse the outer layer of the pass pipeline text and

[PM] Simplify how we parse the outer layer of the pass pipeline text and
remove an extra, redundant pass manager wrapping every run.

I had kept seeing these when manually testing, but it was getting really
annoying and was going to cause problems with overly eager invalidation.
The root cause was an overly complex and unnecessary pile of code for
parsing the outer layer of the pass pipeline. We can instead delegate
most of this to the recursive pipeline parsing.

I've added some somewhat more basic and precise tests to catch this.

llvm-svn: 225253