[Constants] If we already have a ConstantInt*, prefer to use isZero/isOne/isMinusOne...

[Constants] If we already have a ConstantInt*, prefer to use isZero/isOne/isMinusOne instead of isNullValue/isOneValue/isAllOnesValue inherited from Constant. NFCI

Going through the Constant methods requires redetermining that the Constant is a ConstantInt and then calling isZero/isOne/isMinusOne.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307292 91177308-0d34-0410-b5e6-96231b3b80d8