Don't rely on 'l'(ell) modifiers to indicate a label reference

Summary:
It's not necessary to use an 'l'(ell) modifier when referencing a label.
Treat block addresses and MBB references as if the modifier is used
anyway. This prevents us from generating references to ficticious
labels.

Reviewers: jyknight, nickdesaulniers, hfinkel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71849