[DeadStoreElimination] Salvage debug info from dead insts

According to `llvm-dwarfdump --statistics` this salvages 43 additional
unique source variables in a stage2 build of clang. It increases the
size of the .debug_loc section by 0.002% (or 2864 bytes).

Differential Revision: https://reviews.llvm.org/D43220

llvm-svn: 325035