Fix invalid_upcast_comparisons treating non-widening casts as upcasts

[shulaoda]

Fixes #7648

invalid_upcast_comparisons decides a comparison is always true or false by applying the bounds of the pre-cast type to the cast expression, which only holds while the cast preserves the value. numeric_cast_precast_bounds bailed out only when both types had the same size, so a narrowing cast such as u64 as i32, or a sign-losing widening one such as i8 as u64, kept the source bounds. For b: u64, (b as i32) < 0 was reported as always false, but u64::MAX as i32 is -1.

Also println!("{}", 128u16 as i8); -> -128, so (u16 as i8) < 0 is not always false.

The bail-out now keeps only casts to a strictly wider integer type whose signedness can represent the whole source range.

changelog: [invalid_upcast_comparisons]: don't draw conclusions from a cast that is not a widening cast

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