Tweaks to iadd3 with int64, small iadd3 opts and int64 opts
Three things:
- Prevent generation of iadd3 for 64-bit integers. I don't think this is supported on any platform (Intel or otherwise).
- Generate iadd3 in more cases.
- Do some small optimizations before lowering 64-bit integers.
There are a couple more commits that did not have any effect in https://gitlab.freedesktop.org/idr/mesa/-/commits/wip/int64-iadd3.