[RISCV] Lower interleave2 intrinsics to vsseg2

[RISCV] Lower interleave2 intrinsics to vsseg2

This patch teaches the RISCV TargetLowering class to lower interleave
intrinsics to vsseg2, so it can lower interleaved stores for scalable vectors.
Previously, we could only lower stores of interleaves for fixed length vectors
with vector shuffles.

This uses the lowerInterleaveIntrinsic interface for the interleaved
access pass that was added in D146218, and subsumes the DAG combine
approach taken in D144175

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D153864