Fix PR1517:

Fix PR1517:
Use SmallPtrSet instead of std::vector to eliminate duplicate uses in a
function generated with -gen-function. This prevents the output from having
multiple duplicate declarations of constants and gvals.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37616 91177308-0d34-0410-b5e6-96231b3b80d8