[Utils] Salvage debug info in recursive inst deletion

[Utils] Salvage debug info in recursive inst deletion

In stage2 -O3 builds of llc, this results in a 0.3% increase in the
number of variables with locations, and a 0.2% increase in the number of
unique source variables overall.

The size of the .debug_loc section of the llc dsym increases by 0.5%.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326621 91177308-0d34-0410-b5e6-96231b3b80d8