Make salvageDebugInfo of casts work for dbg.declare and dbg.addr

Summary:
Instcombine (and probably other passes) sometimes want to change the
type of an alloca. To do this, they generally create a new alloca with
the desired type, create a bitcast to make the new pointer type match
the old pointer type, replace all uses with the cast, and then simplify
the casts. We already knew how to salvage dbg.value instructions when
removing casts, but we can extend it to cover dbg.addr and dbg.declare.

Fixes a debug info quality issue uncovered in Chromium in
http://crbug.com/784609

Reviewers: aprantl, vsk

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D40042

llvm-svn: 318203