[CodeGen] Use APInt::setLowBits/setHighBits/setBitsFrom in more places

[CodeGen] Use APInt::setLowBits/setHighBits/setBitsFrom in more places

This patch replaces ORs with getHighBits/getLowBits etc. with setLowBits/setHighBits/setBitsFrom.

In a few of the places we weren't ORing, but the KnownZero/KnownOne vectors were already initialized to zero. We exploit this in most places already there were just some that were inconsistent.

Differential Revision: https://reviews.llvm.org/D30965

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297860 91177308-0d34-0410-b5e6-96231b3b80d8