[llvm] Fix for logic in combineExtract() (PR #108208)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

Hmm, I think the intent was for the type of the initial `Op` to be deliberately ignores, as it is supposed to be considered implicitly bitcast to `VecVT`.   But it is true that the routine assumes that `VecVT` must be a byte vector.  One of the two call sites (in combineTruncateExtract) ensures that, but the other call site (in combineEXTRACT_VECTOR_ELT) does not.  So I think the better fix would be to add a check to that call site instead.

https://github.com/llvm/llvm-project/pull/108208