LLVM tweaks/fixes

Summary: - Add -llvm option to prod Makefile, delete some old options.
- Fix a bug in processSvcReqs that was causing us to create an IncomingBranch
  with a nullptr TCA.
- Use emitEphemeralServiceReq in bindjmp translation instead of directly
  calling packServiceReqArgs and emitServiceReqWork. This just cleans up the
  code; it does the same thing at runtime.
- Change not_reached() at base case of codeBlockChoose() to
  always_assert(false). Using not_reached() here is pretty silly since it's a
  catastrophic failure.

Reviewed By: @jasone

Differential Revision: D1751883

Signature: t1:1751883:1419026706:730945ed86bdf7d1d12ba6a8b2b9e0bc2e7ca8dc