[AArch64][GlobalISel][RegBankSelect] Improve rbs of G_BUILD_VECTOR when fed by fp values.

This is actually two changes. One is to avoid copies when fp values are fed into
a build_vector, without being able to tell from the opcode.

The other is that build_vectors are also marked as only defining FP, since they
produce vector results.

Differential Revision: https://reviews.llvm.org/D97968