aarch64: add missing F16 and F128 rules for scalar_size in ISLE

[dsecurity49]

Add missing F16 and F128 rules to scalar_size in inst.isle, as well as their corresponding models in prelude.isle.

Without these, rules that pass f16 or f128 types to scalar_size (like fma.f16) trigger an ISLE panic because scalar_size is a total term.

[cfallin]

We usually have compile-tests with the precise-output flag to show (and lock down) the actual compilation output. Could you include that here? And could you add something for f128 as well if aarch64 has lowerings for it (I don't remember if it does)?

[dsecurity49]

Updated the f16 test to precise-output (emits fmadd h0, h0, h1, h2 as expected).

For f128: there are no native 128-bit float instructions on AArch64, so it would need softfloat libcalls (__addtf3, __fmatf3, etc.) which cranelift doesn't have on any backend yet, even x64 only handles f128const.For now I kept the scalar_size rule for F128 since it prevents a confusing panic, but happy to drop it if you'd prefer to keep this minimal.

[dsecurity49]

After checking further, keeping the F128 rule would be misleading,f128 arithmetic hits unexpected scalar FP operand size (size128)deeper in the backend anyway, so the scalar_size rule for it is effectively dead code. I've removed it and the corresponding model from prelude.isle, keeping only the F16 fix.