Spike to define real math ops and lowering of one variant of add to corresponding integer ops.

The only reason in starting with a fixedpoint add is that it is the absolute simplest variant and illustrates the level of abstraction I'm aiming for.

The overall flow would be:
  1. Determine quantization parameters (out of scope of this cl).
  2. Source dialect rules to lower supported math ops to the quantization dialect (out of scope of this cl).
  3. Quantization passes: [-quant-convert-const, -quant-lower-uniform-real-math, -quant-lower-unsupported-to-float] (the last one not implemented yet)
  4. Target specific lowering of the integral arithmetic ops (roughly at the level of gemmlowp) to more fundamental operations (i.e. calls to gemmlowp, simd instructions, DSP instructions, etc).

How I'm doing this should facilitate implementation of just about any kind of backend except TFLite, which has a very course, adhoc surface area for its quantized kernels. Options there include (I'm not taking an opinion on this - just trying to provide options):
  a) Not using any of this: just match q/dbarrier + tf math ops to the supported TFLite quantized op set.
  b) Implement the more fundamental integer math ops on TFLite and convert to those instead of the current op set.

Note that I've hand-waved over the process of choosing appropriate quantization parameters. Getting to that next. As you can see, different implementations will likely have different magic combinations of specific math support, and we will need the target system that has been discussed for some of the esoteric cases (i.e. many DSPs only support POT fixedpoint).

Two unrelated changes to the overall goal of this CL and can be broken out of desired:
  - Adding optional attribute support to TabelGen
  - Allowing TableGen native rewrite hooks to return nullptr, signalling that no rewrite has been done.

PiperOrigin-RevId: 235267229