[KnownBits] Add methods for determining if the known bits represent a...

[KnownBits] Add methods for determining if the known bits represent a negative/nonnegative number and add methods for changing the negative/nonnegative state

Summary: This patch adds isNegative, isNonNegative for querying whether the sign bit is known. It also adds makeNegative and makeNonNegative for controlling the sign bit.

Reviewers: RKSimon, spatel, davide

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32651

llvm-svn: 301747