Make the Verifier more strict about gc.statepoints

Make the Verifier more strict about gc.statepoints

The recently added documentation for statepoints claimed that we checked the parameters of the various intrinsics for validity.  This patch adds the code to actually do so.  I also removed a couple of redundant checks for conditions which are checked elsewhere in the Verifier and simplified the logic using the helper functions from Statepoint.h.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223259 91177308-0d34-0410-b5e6-96231b3b80d8