[llvm] [SLP]Improve/fix subvectors in gather/buildvector nodes handling (PR #104144)

[Danial Klimkin via llvm-commits]

dklimkin wrote:

We are hitting an assert error with the last reland:

assert.h assertion failed at llvm/lib/IR/Instructions.cpp:1854 in bool isSingleSourceMaskImpl(ArrayRef<int>, int): I >= 0 && I < (NumOpElts * 2) && "Out-of-bounds shuffle mask element" in https://github.com/openxla/xla/blob/main/xla/service/gpu/fusions/triton/triton_fusion_emitter_parametrized_test.cc#L65

I'll see if we can create a simpler reproducer but we may need to revert in the meanwhile.

https://github.com/llvm/llvm-project/pull/104144