[LVI] Exploit nsw/nuw when computing constant ranges

As the title says. Modelled after similar code in SCEV.

This is useful when analysing induction variables in loops which have been canonicalized by other passes. I wrote the tests as non-loops specifically to avoid the generality introduced in http://reviews.llvm.org/D17174. While that can handle many induction variables without *needing* to exploit nsw, there's no reason not to use it if we've already proven it.

Differential Revision: http://reviews.llvm.org/D17177

llvm-svn: 260705