nir: Split NIR_INTRINSIC_TYPE into separate src/dest indices

nir: Split NIR_INTRINSIC_TYPE into separate src/dest indices

We're about to introduce conversion ops which are going to want two
different types.  We may as well just split the one we have rather than
end up with three.  There are a couple places where this is mildly
inconvenient but most of the time I find it to actually be nicer.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6945>