Fix IRBuilder CreateBitOrPointerCast for vector types

Summary:
This function was not taking into account that the
input type could be a vector, and wasn't properly
working for vector types.

This caused an assert when building spec2k6 perlbmk for armv8.

Reviewers: rengolin, mzolotukhin

Subscribers: silviu.baranga, mzolotukhin, rengolin, eugenis, jmolloy, aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D12559

llvm-svn: 246759