Utility function to map a loop on a parametric grid of virtual processors

This CL introduces a simple loop utility function which rewrites the bounds and step of a loop so as to become mappable on a regular grid of processors whose identifiers are given by SSA values.

A corresponding unit test is added.

For example, using CUDA terminology, and assuming a 2-d grid with processorIds = [blockIdx.x, threadIdx.x] and numProcessors = [gridDim.x, blockDim.x], the loop:
```
   loop.for %i = %lb to %ub step %step {
     ...
   }
```
is rewritten into a version resembling the following pseudo-IR:
```
   loop.for %i = %lb + threadIdx.x + blockIdx.x * blockDim.x to %ub
      step %gridDim.x * blockDim.x {
     ...
   }
```

PiperOrigin-RevId: 258945942