[TargetPassConfig] Feature generic options to setup start/stop-after/before

This patch refactors the code used in llc such that all the users of the
addPassesToEmitFile API have access to a homogeneous way of handling
start/stop-after/before options right out of the box.

In particular, just invoking addPassesToEmitFile will set the proper
pipeline without additional effort (modulo parsing a .mir file if the
start-before/after options are used.

NFC.

Differential Revision: https://reviews.llvm.org/D30913

llvm-svn: 309599