[LegalizeVectorOps][AArch64][RISCV][X86] Use OpVT for ISD::SETCC in LegalizeVectorOps.

Previously, LegalizeVectorOps used the result VT while LegalizeDAG
used the operand VT. This patch makes them both use the operand VT.

This also makes it consistent with how the default cost model works.

I've hacked the AArch64 cost model to maintain old behavior for some
f16 vectors.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D149572