Fix ptr vector inconsistency in CreatePointerCast

One form would accept a vector of pointers, and the other did not.
Make both accept vectors of pointers, and add an assertion
for the number of elements.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187464 91177308-0d34-0410-b5e6-96231b3b80d8