Change the keywords for the zext and sext parameter attributes to be
zeroext and signext so they don't conflict with the keywords for the
instructions of the same name. This gets around the ambiguity.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40069 91177308-0d34-0410-b5e6-96231b3b80d8