[BDCE] Salvage debug info from dying insts

This results in 15 additional unique source variables in a stage2 build
of FileCheck (at '-Os -g'), with a negligible increase in the size of
the .debug_loc section.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325660 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed