[SelectionDAG][AArch64][SVE] Perform SETCC condition legalization in LegalizeVectorOps

This is currently performed in SelectionDAGLegalize, here we make it also
happen in LegalizeVectorOps, allowing a target to lower the SETCC condition
codes first in LegalizeVectorOps and then lower to a custom node afterwards,
without having to duplicate all of the SETCC condition legalization in the
target specific lowering.

As a result of this, fixed length floating point SETCC nodes can now be
properly lowered for SVE.

Differential Revision: https://reviews.llvm.org/D98939