compiler assert with >8 bit non-power-of-two-sized member of `@bigendian` bitstruct backed by char array

```c3 bitstruct Foo : char[4] @bigendian { ushort a : 0..14; } fn void main() { Foo f; f.a = 1; } ``` ``` ⚠️ The compiler encountered an unexpected error: "Violated assert: is_power_of_two(bitsize)". - Function: llvm_bswap_non_integral(...) - Source file: /build/way6vk3jjzqavwja3d6q067bjrmpf8xz-source/src/compiler/llvm_codegen_expr.c:951 🙏 Please consider taking the time to file an issue on GitHub (https://github.com/c3lang/c3c/issues/new), so that we can get it fixed. ``` this does not seem to happen when the bitstruct is backed by plain integer types like `uint`.