[DebugInfo] Recover debug intrinsics when killing duplicated/empty basic blocks

When basic blocks are killed, either due to being empty or to being an if.then
or if.else block whose complement contains identical instructions, some of the
debug intrinsics in that block are lost. This patch sinks those intrinsics
into the single successor block, setting them Undef if necessary to
prevent debug info from falling out-of-date.

Differential Revision: https://reviews.llvm.org/D70318