[LPM] Use a map from analysis ID to immutable passes in the legacy pass

manager to avoid a slow linear scan of every immutable pass and on every
attempt to find an analysis pass.

This speeds up 'check-llvm' on an unoptimized build for me by 15%, YMMV.
It should also help (a tiny bit) other folks that are really
bottlenecked on repeated runs of tiny pass pipelines across small IR
files.

llvm-svn: 247240