Fix a disconcerting bug in Value::isUsedInBasicBlock, which gave wrong answers for blocks larger than 3 instrs.

Also add a unit test. PR15727.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179370 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed