For stage1 ZIR instructions and stage1 AIR instructions, the instruction
op code was taking up 8 bytes due to padding even though it only needed
1 byte. This commit reduces the ref_count field from uint32_t to
uint16_t because the code only really cares if instructions are
referenced at all, not how many times they are referenced. With the
ref_count field reduced to uint16_t the uint8_t op code is now placed in
the freed up space.

Empirically, this saves 382 MiB of peak RAM usage when building the
self-hosted compiler, which is a reduction of 5%. Consequently this
resulted in a 3% reduction of cache-misses when building the self-hosted
compiler.

This was @SpexGuy's idea, committed by me because we tested it on my
computer.