[Utils] Salvage debug info of DCE'ed mul/sdiv/srem instructions

Here are the number of additional debug values salvaged in a stage2
build of clang:

  63 SALVAGE: MUL
  1250 SALVAGE: SDIV

(No values were salvaged from `srem` instructions in this experiment,
but it's a simple case to handle so we might as well.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324976 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed