[InstCombine] Use isSignBitCheck to simplify an if statement. Directly create new sign bit compares instead of manipulating the constant. NFCI
[InstCombine] Use isSignBitCheck to simplify an if statement. Directly create new sign bit compares instead of manipulating the constant. NFCI

Since we no longer had the direct constant compares, manipulating the constant seemeded less clear.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@314830 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm