[AArch64][GlobalISel] Use MI::getIntrinsicID in more spots. NFC.

There's technically a difference in the logic used by these
findIntrinsicID and MachineInstr::getIntrinsicID, but it shouldn't
be a meaningful difference here, with G_INTRINSIC instructions.
getIntrinsicID's "first non-def" logic should be correct for those.