Head off invalidations in to_indices for NTuple{N,Int} (#37021)
The most common argument type for `to_indices` is a tuple of `Int`,
yet we rely on the `Integer` fallback. This makes it vulnerable to
invalidation by methods like https://github.com/JuliaMath/Interpolations.jl/blob/bcd05a3f0843661104589c31da8d257fecdbe265/src/Interpolations.jl#L273
which would rather convert non-`Int` integers.
This prevents approximately 80 MethodInstance invalidations.
