pan/bi: FP16 support

Adds initial vectorized FP16 to Bifrost. Draft because there are a few dEQP fails to sort out, there is a big hack to work around INSTR_INVALID_ENC faults on some tests, and codegen is terrible right now (for the 1-bit boolean preliminary, this is a regression -- but we can potentially do much better than master). Still, some of the preliminaries can be landed now at least.

int16 will need quite a bit more work and can come later.