Types and alignment for the 0xFE opcode space.

Ask it for {Kind, ValueType, Width} for any atomic instruction, where Width is the access in bytes. The interpreter reads the same answer, so the two cannot drift apart.

The read-modify-write family is written out rather than derived from the instruction's name. Parsing the name would be shorter, but this function is consulted for every memory instruction in a module, including the ordinary loads and stores of a module with no atomics in it, and a fallthrough that builds a string per instruction is not something to put on the path that validates and instantiates every module.

Alignment is exact here, not a hint. An ordinary load may declare any alignment up to the natural one and the number is only advice; an atomic access must declare exactly its natural alignment, and a module that declares anything else is invalid rather than merely slow. That is what lets an implementation assume the access does not straddle two words.