Make fast inverse and fast inverse sqrt intrinsics instead.

They were coded for acceptable accuracy using a Newton step. This didn't get
the special values right (1/0 was not inf), and meant that they could
sometimes be slower than true division due to increased instruction latency!

I've removed the Newton step, and made them explicitly selected intrinsics
instead of something that triggers automatically.
Add gpu test and opencl implementation for fast inverse
Fix simd_op_check for new fast_inverse
Add newton update back to arm approx inverse

Otherwise it's way too inaccurate.
Another simd_op_check fix.
Merge pull request #519 from halide/fast_inverse

Fast inverse