[AArch64][CodeGen] Restrict bfloat vector operations to what's actually supported

Previously in addTypeForNeon, we would set the operations for bfloat vectors
like other generic types. But as bfloat is a storage-only type a number of
operations shouldn't be set. This patch fixes that.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D85101