Rollup merge of #100653 - cuviper:fptoint_sat, r=michaelwoerister,antoyo

Rollup merge of #100653 - cuviper:fptoint_sat, r=michaelwoerister,antoyo

Move the cast_float_to_int fallback code to GCC

Now that we require at least LLVM 13, that codegen backend is always
using its intrinsic `fptosi.sat` and `fptoui.sat` conversions, so it
doesn't need the manual implementation. However, the GCC backend still
needs it, so we can move all of that code down there.