[llvm] Reland "[VectorCombine] Combine adjacent loads feeding as shuffle operands" (PR #229662)

[Thurston Dang via llvm-commits]

thurstond wrote:

> Have you tested this reland with a UBSan-enabled clang? [8d05ee1](https://github.com/llvm/llvm-project/commit/8d05ee14ac96ed82ffef778f4c6684ded7d2911d) only fixes one overflow. In #229306 I had added fixes in two more places (and added another test case).

Non-UBSan presubmit is failing (https://github.com/llvm/llvm-project/actions/runs/37577968281/job/112651004719?pr=229662#logs):

```
  # | opt: /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/IR/Instructions.cpp:1966: bool isSingleSourceMaskImpl(ArrayRef<int>, int): Assertion `I >= 0 && I < (NumOpElts * 2) && "Out-of-bounds shuffle mask element"' failed.
```

which is one of the extra cases that was handled in https://github.com/llvm/llvm-project/pull/229306.


https://github.com/llvm/llvm-project/pull/229662