[BPF] fix a use after free bug Commit fff2721286e1 ("[BPF] Fix CO-RE bugs with bitfields") fixed CO-RE handling bitfield issues. But the implementation introduced a use after free bug. The "Base" of the intrinsic might be freed so later on accessing the Type of "Base" might access the freed memory. The failed test case, CodeGen/BPF/CORE/offset-reloc-middle-chain.ll is exactly used to test such a case. Similarly to previous attempt to remember Metadata etc, remember "Base" pointee Alignment in advance to avoid such use after free bug.