Revert "[InstCombine] Canonicalize `(sitofp x)` -> `(uitofp x)` if `x >= 0`"

This reverts commit d80d5b92.

It wasn't a particularly important transform to begin with and caused
some codegen regressions on targets that prefer `sitofp` so dropping.

Might re-visit along with adding `nneg` flag to `uitofp` so its easily
reversable for the backend.