Steer round to even to Z's hardware operation (for Z), and MLIR/LLVM roundEven for the other platforms.

[AlexandreEichenberger]

Round to nearest even was currently implemented in software:

  // Use numpy algorithm for rint as follows.
  // ```
  // double y, r;
  // y = npy_floor(x);
  // r = x - y;
  //
  // if (r > 0.5) {
  //     y += 1.0;
  // }
  //
  // /* Round to nearest even */
  // if (r == 0.5) {
  //     r = y - 2.0*npy_floor(0.5*y);
  //     if (r == 1.0) {
  //         y += 1.0;
  //     }
  // }

This can be replaced by one instruction on many architectures (e.g. on Z by FIEBRA or VFISB). This PR does that for z machines.

For others, MLIR & LLVM actually implement a math::round_even that leads to efficient instructions as well. At this time, this is not enabled for Z, thus the direct use of ASM for the Z platform. If the ASM cannot be used on Z, then the math::round_even_emulation (algorithm above) performs better than the default LLVM function call backup. For other machines, the LLVM round even is used.

Another issue with using ASM driven directive is that we cannot use the LLVM features that let us used SIMD vectors of arbitrary lengths. I baked this support in the ONNX to Krnl lowering using vector::ShapeCastOp and then extract and insert. That same approach can be used elswhere. I checked the HW that the generated shuffle are eventually all gone (as we shuffle entire vectors). Right now, it is handled in the ONNX to KRNL. It could be migrated to Krnl to LLVM, but would require more work that I am not too familiar with.

[tungld]

LGTM.

[tungld]

This else looks redundant since we have return inside the if.

[AlexandreEichenberger]

Tx, will fix

[jenkins-droid]

Jenkins Linux amd64 Build #15816 [push] Steer round to even to Z... started at 07:39

[jenkins-droid]

Jenkins Linux ppc64le Build #14846 [push] Steer round to even to Z... started at 08:51

[jenkins-droid]

Jenkins Linux s390x Build #15819 [push] Steer round to even to Z... started at 08:39

[jenkins-droid]

Jenkins Linux amd64 Build #15816 [push] Steer round to even to Z... passed after 1 hr 13 min

[jenkins-droid]

Jenkins Linux s390x Build #15819 [push] Steer round to even to Z... passed after 1 hr 35 min

[jenkins-droid]

Jenkins Linux ppc64le Build #14846 [push] Steer round to even to Z... passed after 2 hr 15 min