[ValueTracking] Use APInt::isNegative instead of using operator[BitWidth-1]. NFCI
[ValueTracking] Use APInt::isNegative instead of using operator[BitWidth-1]. NFCI

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@298584 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 31f7be5e2b59e9fe74ce9d4a4e4b843cf94ac132)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm