[X86][BF16] Share FP16 vector ABI with BF16

[pengfei]

Summary

The ABI of BF16 is identical to FP16 rather than i16.

Fixes #62997

[RKSimon]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

642

Do we need a TLI callback / legality check for f16?

[pengfei]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

642

I think not. The PartVT comes from TLI.getRegisterTypeForCallingConv. We don't need to add another callback and targets use this way should have already made f16 legal.
I can add an TLI.isTypeLegal(PartVT) if you wish.

[pengfei]

Add assert for type legal.

[RKSimon]

LGTM