[RISCV] Add new entry points to getContainerForFixedLengthVector

While working on adding fixed-length vectors to the calling convention,
it was necessary to be able to query for a fixed-length vector container
type without access to an instance of SelectionDAG.

This patch modifies the "main" getContainerForFixedLengthVector function
to use an instance of TargetLowering rather than SelectionDAG, and
preserves the SelectionDAG overload as a wrapper.

An additional non-static version of the function was also added to
simplify the common case in RISCVTargetLowering.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D97925