[AArch64] Clarify atomic load/store size condition (NFCI) (#91907)

This is currently bailing out on MemSizeInBytes larger than 64 bytes.
However, the following code can only handle sizes up to 8 bytes.
Possibly there was confusion here between MemSizeInBytes and
MemSizeInBits.

I *think* that this can't actually result in an out of bounds read of
the opcode table because we'll only ever mark loads/stores of up to 8
bytes as legal (16 byte atomics are custom-legalized earlier). As such,
I've changed this condition to an assert.