microsoft/compiler: Better and simpler bitcast reduction

microsoft/compiler: Better and simpler bitcast reduction

Using nir_gather_ssa_types works much better. There's 2 differences
compared to what I was doing before:
1. Multiple passes to allow data to propagate forward and backward
   through the whole shader.
2. Allowing a value to have indeterminate types due to having both
   int and float usages.

So this deletes some code and gets better results. Wish I'd known
this existed last week.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23062>