llc: Rework -run-pass option

We now construct a custom pass pipeline instead of injecting
start-before/stop-after into the default pipeline construction. This
allows to specify any pass known to the pass registry. Previously
specifying indirectly added analysis passes or passes not added to the
pipeline add all would not be added and we would silently do nothing.

This also restricts the -run-pass option to cases with .mir input.

llvm-svn: 269003