[RISCV] Fix misuse of EVT::changeVectorElementType() in legalizeScatterGatherIndexType. (#165829)
This function doesn't work well when the type is a SimpleVT, but the
changed type isn't. We need an LLVMContext to make an non-SimpleVT, but
there's nowhere to get it from.
Fix this by using EVT::getVectorVT instead.
In the added test, v7i8 is a SimpleVT, but v7i64 is not.