[BPF] fix a bug in __builtin_preserve_field_info() with FIELD_BYTE_SIZE

During deriving proper bitfield access FIELD_BYTE_SIZE,
function Member->getStorageOffsetInBits() is used to
get llvm IR type storage offset in bits so that
the byte size can permit aligned loads/stores with previously
derived FIELD_BYTE_OFFSET.

The function should only be used with bitfield members and it will
assert if ASSERT is turned on during cmake build.
  Constant *getStorageOffsetInBits() const {
    assert(getTag() == dwarf::DW_TAG_member && isBitField());
    if (auto *C = cast_or_null<ConstantAsMetadata>(getExtraData()))
      return C->getValue();
    return nullptr;
  }

This patch fixed the issue by using Member->isBitField()
directly and a test case is added to cover this missing case.
This issue is discovered when running Andrii's linux kernel CO-RE
tests.

Differential Revision: https://reviews.llvm.org/D69761