`Base.ssqs`: fix small type instability (#60896)
`m` is of floating-point type, while `k` is an integer.
This change makes the type of the branch expression always be a subtype
of `Integer`, and indeed, a concrete `Int` in practically important
cases.
This fixes a tiny type instability. It is of the small union kind,
usually addressed well by union splitting in the Julia compiler.
However, it seems that this type instability had caused issues for some
GPU targets in the past, see discussion on linked PR.
This PR is a simplification of the PR #54869.
