[SPIR-V] Legalize G_PHI of oversized vectors via fewer-elements (#203993)
`G_PHI` on vectors wider than the SPIR-V max vector size previously
failed legalization. This PR adds a `fewerElementsIf` rule that splits
them down to `MaxVectorSize`, matching how other vector ops are handled
in `SPIRVLegalizerInfo.cpp`.
Added the following test
`llvm/test/CodeGen/SPIRV/instructions/phi-large-vector.ll` covering
spirv32 and spirv64.