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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@222092 91177308-0d34-0410-b5e6-96231b3b80d8

dslab-epfl/asap,GPUOpen-Drivers/llvm,dslab-epfl/asap,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,dslab-epfl/asap,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm