[KnownBits] Add zext, sext, and trunc methods to KnownBits

[KnownBits] Add zext, sext, and trunc methods to KnownBits

This patch adds zext, sext, and trunc methods to KnownBits and uses them where possible.

Differential Revision: https://reviews.llvm.org/D32784

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302088 91177308-0d34-0410-b5e6-96231b3b80d8