Add support to the pattern match library for matching NSW and NUW

instructions. I needed this for a quick experiment I was making, and
while I've no idea if that will ever get committed, I didn't want to
throw away the pattern match code and for anyone else to have to write
it again. I've added unittests to make sure this works correctly.

In fun news, this also uncovered the IRBuilder bug. Doh!

llvm-svn: 198541