The current implementation of lowering to llvm for vector.extract
incorrectly assumes that if the number of indices is zero, the operation
can be folded away. This PR removes this condition and relies on the
folder to do it instead.
This PR also unifies the logic for scalar extracts and slice extracts,
which as a side effect also enables vector.extract lowering for n-d
vector.extract with dynamic inner most dimension. (This was only
prevented by a conservative check in the old implementation)
a8f927161b