Never consider int and float vars for `FnPtr` candidates
This solves a regression where `0.0.cmp()` was ambiguous when a custom trait with a `cmp` method was in scope.
For integers it shouldn't be a problem in practice so I wasn't able to add a test.
I'm not sure whether there could be more issues hidden in the shadows as mentioned in the issue, but this should at least fix the problematic regression immediately.
fixes#109892
r? oli-obk
7d3207be97