Crash when concrete non-undef structs contain floats

[Sebastian Poeplau]

The code that we introduced to fix #134 doesn't handle floats correctly; it crashes trying to bswap an expression of floating-point kind. We need to convert the expression to bit-vector kind before using it; the same is likely true for Booleans (just like in the symbolic handling of store, so let's reuse the function we introduced there).