[MachineLICM] Assert that locations from debug insts are not lost

Summary:
Assert that MachineLICM does not move a debug instruction and then drop
its debug location. Later passes require each debug instruction to have
a location.

Testing: check-llvm, clang stage2 RelWithDebInfo build (x86_64)

Reviewers: aprantl, davide, chrisjackson, jmorse

Subscribers: hiraditya, asbirlea, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80665