[llvm] [AMDGPU] Fix an issue that wrong index is used in calculation of byte provider when the op is extract_vector_elt (PR #91697)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

> Why do we want to differentiate them here?

We don't gain anything from looking into the vectors for scalarsize >= 32. calculateSrcByte will just return the Op since vector handling hasn't been built yet, and then we will need to re-extract the dword when building the perm https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AMDGPU/SIISelLowering.cpp#L12193  -- by disallowing them, we avoid some unecessary computation. Additionally, it does not introduce the regression mentioned above. 

https://github.com/llvm/llvm-project/pull/91697