[X86] Skip AMX type lowering when AMX is not used (#92910)

The pass iterates over the IR multiple times, but most code doesn't use
AMX. Therefore, do a single iteration in advance to check whether a
function uses AMX at all, and exit early if it doesn't. This makes the
function-has-AMX path slightly more expensive, but AMX users probably
care a lot less about compile time than JIT users (which tend to not use
AMX).

For us, it reduces the time spent in this pass from 0.62% to 0.12%.

Ideally, we wouldn't even need to iterate over the function to determine
that it doesn't use AMX.