BumpPtrAllocator: remove 'no slabs allocated yet' check

We already handle the no-slabs case when checking whether the current slab
is large enough: if no slabs have been allocated, CurPtr and End are both 0.
alignPtr(0), will still be 0, and so "if (Ptr + Size <= End)" fails.

Differential Revision: http://reviews.llvm.org/D4943