[llvm] [GlobalISel] Fix miscompile when narrowing vector load/stores to non-byte-sized types (PR #136739)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

It would be nice if this worked and didn't fall-back to SDAG by packing the bits, but LGTM for the fix.

https://github.com/llvm/llvm-project/pull/136739