[NVPTX] Support float <-> 2 x half bitcasts

Make sure NVPTX backend can handle bitcasting between `float` and `<2 x half>` types.

This was discovered through: https://github.com/intel/llvm/issues/5969
I'm not suggesting that such bitcasts make much sense, but it feels like the compiler should not hard crash on them.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D124171