[ConstantInt] Use ConstantInt::getValue instead of Constant::getUniqueInteger...

[ConstantInt] Use ConstantInt::getValue instead of Constant::getUniqueInteger in a few places where we obviously have a ConstantInt. NFC

getUniqueInteger will ultimately call ConstantInt::getValue, but calling ConstantInt::getValue should be inlined.

llvm-svn: 310069