Instead of using the `istore8` special store opcodes, perform a ireduce and then store.

These instructions are essentially a combination of a `ireduce+store`, so remove the special store, and start recognizing that idiom in the backends.

Since `ireduce` is a no-op from the point of view of the backends, we can simply delete those lowering rules. they don't add any novel instructions other than just separately lowering both `ireduce` and `store`.

There are a lot of s390x tests delete here, I've checked and there also exist equivalent `store` based tests for those, so no point in keeping them around.

These are going to be removed in follow up commits