CommandFlags.h/llc: Move StopAfter/StartBefore options to llc.

Move those two options to llc:

The options in CommandFlags.h are shared by dsymutil, gold, llc,
llvm-dwp, llvm-lto, llvm-mc, lto, opt.

-stop-after/-start-after only affect codegen passes however only gold and llc
actually create codegen passes and I believe these flags to be only
useful for users of llc. For the other tools they are just highly
confusing: -stop-after claims to "Stop compilation after a specific
pass" which is not true in the context of the "opt" tool.

Differential Revision: https://reviews.llvm.org/D23050

llvm-svn: 277551