[BPF] Fix dst/val mismatch in class ATOMIC_NOFETCH (#107288)
All ATOMIC_NOFETCH insns have "$dst = $val" constraints. So let us
enforce "$dst = $val" having the same register type in ATOMIC_NOFETCH as
well.
Currently, things work since ATOMIC_NOFETCH does not have source code
pattern matching. I am experimenting to introduce memory ordering to
BPFInstrInfo.td file and pattern matching will be needed. Eventually,
for atomic_fetch_*() insns locked insns could be generated if memory
ordering is memory_order_relaxed.
[1] https://lore.kernel.org/bpf/7b941f53-2a05-48ec-9032-8f106face3a3@linux.dev/