As noted when #82404 was pushed (canonicalizing `sitofp` -> `uitofp`),
different signedness on fp casts can have dramatic performance
implications on different backends.
So, it makes to create a reliable means for the backend to pick its
cast signedness if either are correct.
Further, this allows us to start canonicalizing `sitofp`- > `uitofp`
which may easy middle end analysis.
Closes#86141
9170e38575