Add a kernel usable as a GEBP inner loop for an LLVM IR GEMM

This is not used in any real code path, but I've added an escape hatch that runs
regular matrix multiplies through this kernel for testing purposes.

As far as I can tell this is functionally correct, but I don't yet have a proper
apples-to-apples performance comparison -- that'll have to wait till the
implementation is complete.

PiperOrigin-RevId: 197422075