Fix fuzzer nondeterminism (#8886)

Fixes #8770. The fuzzer was non-deterministic across architectures since
we had two calls to `pick()` in the arguments to `makeAtomicRMW`, so the
evaluation order of the two `pick` calls was non-deterministic. Fix this
by pulling the function calls outside of the arguments of
`makeAtomicRMW`.

Also increase the fuzz metrics test seed file size (2214 bytes -> 1020
KB) to increase the likelihood of catching similar issues in CI, and
exercise more instructions in the test. This bug wasn't caught by CI in
#8253.

Tested by basing #8885 on top of this which failed previously due to the
non-determinism, and now succeeds.