[BPF] Avoid checking for intrinsics using string comparisons. NFC

Use a dyn_cast<> to IntrinsicInst and an enum compare instead.
While touching this code also re-generate the test to use positive check
lines instead of negative ones and remove some unneeded metadata.

Reviewed By: yonghong-song

Differential Revision: https://reviews.llvm.org/D138565