[SLP]Use original vector if need to shuffle truncated root.

If the root scalar is mapped to to the smallest bit width, the vector is
truncated and the types between original buildvector and extracted value
mismatched. For extract, we emit sext/zext instructions, for shuffles we
can reuse oringal vector instead of the truncated one.

Differential Revision: https://reviews.llvm.org/D127974