[llvm] [SLP]Improve minbitwidth analysis. (PR #78976)

[James Y Knight via llvm-commits]

jyknight wrote:

I see it was already reverted.

But prior to that, this change has also triggered a miscompile in xxhash, with llvm/unittests/Support/xxhashTest.cpp failing to compute correct hashes for 6 of its test-cases. (This error showed up in our downstream build, which is building llvm for x86-64, but uses various cpu tuning and other build flags. I don't know if any upstream buildbots observed the bug.)

```
third_party/llvm/llvm-project/llvm/unittests/Support/xxhashTest.cpp:48: Failure
Expected equality of these values:
  uint64_t(0x9013fb74ca603e0c)
    Which is: 10381918045049208332
  xxh3_64bits(ArrayRef(a, size_t(33)))
    Which is: 15799937386795546273

third_party/llvm/llvm-project/llvm/unittests/Support/xxhashTest.cpp:49: Failure
Expected equality of these values:
  uint64_t(0xfa5271fcce0db1c3)
    Which is: 18037604788174959043
  xxh3_64bits(ArrayRef(a, size_t(64)))
    Which is: 4052662859111881885

third_party/llvm/llvm-project/llvm/unittests/Support/xxhashTest.cpp:50: Failure
Expected equality of these values:
  uint64_t(0x79c42431727f1012)
    Which is: 8774177768817496082
  xxh3_64bits(ArrayRef(a, size_t(65)))
    Which is: 15228745306148528873

third_party/llvm/llvm-project/llvm/unittests/Support/xxhashTest.cpp:51: Failure
Expected equality of these values:
  uint64_t(0x591ee0ddf9c9ccd1)
    Which is: 6421817362660052177
  xxh3_64bits(ArrayRef(a, size_t(96)))
    Which is: 9533022609644079181

third_party/llvm/llvm-project/llvm/unittests/Support/xxhashTest.cpp:52: Failure
Expected equality of these values:
  uint64_t(0x8ffc6a3111fe19da)
    Which is: 10375284400542587354
  xxh3_64bits(ArrayRef(a, size_t(97)))
    Which is: 16638996938277853399

third_party/llvm/llvm-project/llvm/unittests/Support/xxhashTest.cpp:53: Failure
Expected equality of these values:
  uint64_t(0x06a146ee9a2da378)
    Which is: 477741026080826232
  xxh3_64bits(ArrayRef(a, size_t(128)))
    Which is: 5146516666195395268
```



https://github.com/llvm/llvm-project/pull/78976