[CodeGen] Emit aliasing metadata for new arrays.

Ensure that all array base pointers are assigned before generating
aliasing metadata by allocating new arrays beforehand.

Before this patch, getBasePtr() returned nullptr for new arrays because
the arrays were created at a later point. Nullptr did not match to any
array after the created array base pointers have been assigned and when
the loads/stores are generated.

llvm-svn: 305675