[GIsel][LegalizerHelper] Account for debug insts when creating mem libcalls [5/14]

Summary:
While lowering memory intrinsics, GIsel attempts to form a tail call to
a library routine.

There might be a DBG_LABEL or something after the intrinsic call,
though: in that case, GIsel should still be able to form the tail call,
and should also delete the debug insts after the tail call as the
transform makes them invalid.

Reviewers: dsanders, aemerson

Subscribers: hiraditya, aprantl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78335