[RISCV] Only expose one interface for getContainerForFixedLengthVector in the...

[RISCV] Only expose one interface for getContainerForFixedLengthVector in the RISCVTargetLowering class

We can have RISCVISelDAGToDAG.cpp call the VT only version by
finding the RISCVTargetLowering object via the Subtarget.

Make the static versions just global static functions in
RISCVISelLowering that can be called by static functions in that
file.