[SCFToCalyx] buildLibraryOp cast floating point to integer types

[jiahanxie353]

In buildLibraryOp, we cast floating point types to integer types with the same width, as discussed here: #7762

The testing case is just the result of running the -arith-expand pass on the following input:

module {
  func.func @main(%b: f64, %c: f64) -> f64 {
    %a = arith.maximumf %b, %c : f64
    return %a : f64
  }
}

[cgyurgyik]

Some small nits :)

[jiahanxie353]

Thanks @cgyurgyik !

Made some changes based on your feedback; also changed convIndex to a more generic function by utilizing the toBitVector function.

[cgyurgyik]

Awesome, thanks!