[llvm] [SelectionDAG] Add support for the 3-way comparison intrinsics [US]CMP (PR #91871)

[Nikita Popov via llvm-commits]

nikic wrote:

```llvm
define <2 x i16> @test_ucmp.2.8.16(<2 x i8> %x, <2 x i8> %y) {
  %1 = call <2 x i16> @llvm.ucmp(<2 x i8> %x, <2 x i8> %y)
  ret <2 x i16> %1
}
```
Asserts:
```
llc: /home/npopov/repos/llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp:7434: SDValue llvm::SelectionDAG::getNode(unsigned int, const SDLoc &, EVT, SDValue, SDValue, SDValue, const SDNodeFlags): Assertion `(VT.isScalableVector() != N2VT.isScalableVector() || VT.getVectorMinNumElements() >= N2VT.getVectorMinNumElements()) && "Insert subvector must be from smaller vector to larger vector!"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	Program arguments: build/bin/llc test.ll
1.	Running pass 'Function Pass Manager' on module 'test.ll'.
2.	Running pass 'X86 DAG->DAG Instruction Selection' on function '@test_ucmp.2.8.16'
 #0 0x0000000006474828 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (build/bin/llc+0x6474828)
 #1 0x00000000064723ee llvm::sys::RunSignalHandlers() (build/bin/llc+0x64723ee)
 #2 0x0000000006474ed8 SignalHandler(int) Signals.cpp:0:0
 #3 0x00007f2303c53710 __restore_rt (/lib64/libc.so.6+0x40710)
 #4 0x00007f2303cab144 __pthread_kill_implementation (/lib64/libc.so.6+0x98144)
 #5 0x00007f2303c5365e gsignal (/lib64/libc.so.6+0x4065e)
 #6 0x00007f2303c3b902 abort (/lib64/libc.so.6+0x28902)
 #7 0x00007f2303c3b81e _nl_load_domain.cold (/lib64/libc.so.6+0x2881e)
 #8 0x00007f2303c4b977 (/lib64/libc.so.6+0x38977)
 #9 0x0000000006249fd7 llvm::SelectionDAG::getNode(unsigned int, llvm::SDLoc const&, llvm::EVT, llvm::SDValue, llvm::SDValue, llvm::SDValue, llvm::SDNodeFlags) (build/bin/llc+0x6249fd7)
#10 0x00000000062f4a41 llvm::DAGTypeLegalizer::WidenVecRes_CMP(llvm::SDNode*) LegalizeVectorTypes.cpp:0:0
#11 0x00000000062ed2bd llvm::DAGTypeLegalizer::WidenVectorResult(llvm::SDNode*, unsigned int) LegalizeVectorTypes.cpp:0:0
#12 0x0000000006294fdb llvm::DAGTypeLegalizer::run() LegalizeTypes.cpp:0:0
```

I'm using this script to generate test cases:
```php
<?php

function makeType(int $numElems, int $bw): string {
  $ty = 'i' . $bw;
  if ($numElems != 0) {
    return '<' . $numElems . ' x ' . $ty . '>';
  }
  return $ty;
}

$bws = [1, 2, 3, 4, 7, 8, 9, 15, 16, 17, 31, 32, 33, 63, 64, 65, 127, 128, 129, 255, 256, 257];

foreach (['ucmp', 'scmp'] as $intrin) {
  for ($numElems = 0; $numElems < 100; ++$numElems) {
    foreach ($bws as $argBW) {
      foreach ($bws as $resBW) {
        if ($resBW == 1) {
          continue;
        }
        $argType = makeType($numElems, $argBW);
        $resType = makeType($numElems, $resBW);
        echo "define $resType @test_$intrin.$numElems.$argBW.$resBW($argType %x, $argType %y) {\n";
        echo "  %1 = call $resType @llvm.$intrin($argType %x, $argType %y)\n";
        echo "  ret $resType %1\n";
        echo "}\n";
        echo "\n";
      }
    }
  }
}
```

https://github.com/llvm/llvm-project/pull/91871