[DirectX] Legalize i8 allocas (#137399)
fixes #137202
investingating i8 allocas I came to find some missing instructions from
out i8 legalization around load, store, and select.
Added those three.
To do i8 allocas right though we needed to walk the uses and find the
casts.
After finding the casts I chose to pick the smallest cast as the cast to
transform to. That would then let me preserve the larger casts that come
later
