integer-division buildifier warning triggers on floating point division

The buildifier warning for integer division seems to trigger when it finds [any binary expression that uses the `/` operator](https://github.com/bazelbuild/buildtools/blob/f2aed9ee205d62d45c55cfabbfd26342f8526862/warn/warn_operation.go#L104-L107), regardless of its operands' types. Ideally, it shouldn't do that when one of the operands is known to be a float, such as a floating point literal or something wrapped with a call to `float`.

A great reproducer for this is to run buildifier linting on a file containing the expression `x = 1.0 / 1.0`.

Start in warn/warn_operation.go at the linked integer-division warning logic, then reproduce the report by running buildifier linting on a file containing x = 1.0 / 1.0. Done means division with a known floating-point operand, including a floating-point literal or a value wrapped with float, no longer triggers the warning while integer division behavior remains covered.

Written by the indexing model from the issue text.