[Utils] Salvage debug info in block simplification

In stage2 -O3 builds of llc, this results in small but measurable
increases in the number of variables with locations, and in the number
of unique source variables overall.

(According to llvm-dwarfdump --statistics, there are 123 additional
variables with locations, which is just a 0.006% improvement).

The size of the .debug_loc section of the llc dsym increases by 0.004%.

llvm-svn: 326629