[RISCV][NFC] Share interleave mask checking logic

This adds two new methods to ShuffleVectorInst, isInterleave and
isInterleaveMask, so that the logic to check if a shuffle mask is an
interleave can be shared across the TTI, codegen and the interleaved
access pass.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D145971