Missing fp16 type validation

[kripken]

It looks like e.g. the validator does not look inside SIMDTernary for e.g. RelaxedNmaddVecF16x8 and require the fp16 feature. That causes the fuzzer to not realize a testcase depends on that feature. In particular, that means that on ClusterFuzz we are sometimes using fp16 without wanting to, which makes those testcases fail in V8 unhelpfully, with errors like

CompileError: WebAssembly.Module(): Compiling function #33 failed: invalid simd opcode: 0xfd14f, enable with --experimental-wasm-fp16 @+3171

ClusterFuzz ignores those silently. Our local fuzzer errors on this, though rarely, so this is not urgent to fix, but it would be good to. Or is V8 ready for fp16 fuzzing perhaps? cc @brendandahl