Fix a bug in IRBuilder that's been there for who knows how long. It
failed to correctly propagate the NUW and NSW flags to the constant
folder for two instructions. I've added a unittest to cover flag
propagation for the rest of the instructions and constant expressions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198538 91177308-0d34-0410-b5e6-96231b3b80d8