[SelectionDAG] Fix illegal widening of scalable-vector loads

The process of widening simple vector loads attempts to use a load of a
wider vector type if the original load is sufficiently aligned to avoid
memory faults.

However this optimization is only legal when performed on fixed-length
vector types. For scalable vector types this is invalid (unless vscale
happens to be 1).

This patch does increase the likelihood of compiler crashes (from
`FindMemType` failing to find a suitable type) but this now better
matches how widening non-simple loads, insufficiently-aligned loads, and
scalable-vector stores are handled.

Patches will be introduced later by which loads and stores can be
widened on targets with support for masked or predicated operations.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D111885