[BPF] fix a use after free bug

Commit fff27212 ("[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.