[Utils] Salvage debug info of DCE'ed shl/lhsr/ashr instructions

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

  1912 SALVAGE: ASHR
   405 SALVAGE: LSHR
   249 SALVAGE: SHL

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324975 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed