[WebAssembly] Avoid `bit_cast` when printing f32 and f64 immediates

Use `APInt` to convert a 32-bit or 64-bit immediate to an `APFloat` rather than
`bit_cast` to a `float` or `double` to avoid going through host floating-point and
potentially changing the bit pattern of NaNs.

Differential Revision: https://reviews.llvm.org/D97490