[RISCV] Make lowerVECTOR_SHUFFLEAsVSlidedown follow source until not EXTRACT_SUBVECTOR.

Current lowerVECTOR_SHUFFLEAsVSlidedown only seeks whether input are
EXTRACT_SUBVECTOR and their source are same. The commit will make the
function seek input and their source until they are not
EXTRACT_SUBVECTOR.

Differential Revision: https://reviews.llvm.org/D138025