[FastISel] Lower `llvm.dbg.value(undef, ...` correctly.

Summary:
Instead of just dropping them.

<rdar://problem/58657146>

Reviewers: aprantl, vsk, ab, paquette, echristo

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72877