[RISCV] Transform unaligned RVV vector loads/stores to aligned ones

This patch adds support for loading and storing unaligned vectors via an
equivalently-sized i8 vector type, which has support in the RVV
specification for byte-aligned access.

This offers a more optimal path for handling of unaligned fixed-length
vector accesses, which are currently scalarized. It also prevents
crashing when `LegalizeDAG` sees an unaligned scalable-vector load/store
operation.

Future work could be to investigate loading/storing via the largest
vector element type for the given alignment, in case that would be more
optimal on hardware. For instance, a 4-byte-aligned nxv2i64 vector load
could loaded as nxv4i32 instead of as nxv16i8.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D104032