Fix a hang when lowering __builtin_dynamic_object_size

If the ObjectSizeOffsetEvaluator fails to fold the object size call, then it may
litter some unused instructions in the function. When done repeatably in
InstCombine, this results in an infinite loop. Fix this by tracking the set of
instructions that were inserted, then removing them on failure.

rdar://49172227

Differential revision: https://reviews.llvm.org/D60298

llvm-svn: 358146