InstSimplify: Optimize ICmpInst xform that uses computeKnownBits

InstSimplify: Optimize ICmpInst xform that uses computeKnownBits

A few things:
- computeKnownBits is relatively expensive, let's delay its use as long
  as we can.
- Don't create two APInt values just to run computeKnownBits on a
  ConstantInt, we already know the exact value!
- Avoid creating a temporary APInt value in order to calculate unary
  negation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222092 91177308-0d34-0410-b5e6-96231b3b80d8