simd_op_check_arm does no checking of float16 instructions

[abadams]

We have pattern matching for some things in CodeGen_ARM.cpp, but no testing of the actual instruction selection!

[steven-johnson]

float_16_t_neon_op_check apaprently does some of this but seems like this code belongs inn simd_op_check instead.

See also https://github.com/halide/Halide/issues/7979