Why does adding inline assembly comments cause such radical change in GCC’s generated code?

The interactions with optimisations are explained about halfway down the “Assembler Instructions with C Expression Operands” page in the documentation.

GCC doesn’t try to understand any of the actual assembly inside the asm; the only thing it knows about the content is what you (optionally) tell it in the output and input operand specification and the register clobber list.

In particular, note:

An asm instruction without any output operands will be treated identically to a volatile asm instruction.

and

The volatile keyword indicates that the instruction has important side-effects […]

So the presence of the asm inside your loop has inhibited a vectorisation optimisation, because GCC assumes it has side effects.