[GVN] Salvage debug info from dead insts

This preserves an additional 581 unique source variables in a stage2
build of clang (according to `llvm-dwarfdump --statistics`). It
increases the size of the .debug_loc section by 0.1% (or 87139 bytes).

Differential Revision: https://reviews.llvm.org/D43255

llvm-svn: 325063