[SelectionDAG] Update getNode asserts for EXTRACT/INSERT_SUBVECTOR.

Summary:
The description of EXTACT_SUBVECTOR and INSERT_SUBVECTOR has been
changed to accommodate scalable vectors (see ISDOpcodes.h). This
patch updates the asserts used to verify these requirements when
using SelectionDAG's getNode interface.

This patch introduces the MVT function getVectorMinNumElements
that can be used against fixed-length and scalable vectors when
only the known minimum vector length is required.

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80709