[mlir][vector] Add gather lowering patterns

This is for targets that do not support gather-like ops, e.g., SPIR-V.

Gather is expanded into lower-level vector ops with memory accesses
guarded with `scf.if`.

I also considered generating `vector.maskedload`s, but decided against
it to keep the `memref` and `tensor` codepath closer together. There's a
good chance that if a target doesn't support gather it does not support
masked loads either.

Issue: https://github.com/llvm/llvm-project/issues/60905

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D145942