BumpPtrAllocator: don't accept 0 for the alignment parameter

It seems unnecessary to have to use an extra branch to check for this special case.

http://reviews.llvm.org/D4945

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216036 91177308-0d34-0410-b5e6-96231b3b80d8