[mlir][Vector] Disable 'vector.extract' folding for unsupported 0-D vectors

The `vector.extract` folding patterns do not support 0-D vectors
(actually, 0-D vector support couldn't even be implemented as a folding
pattern as it would require replacing `vector.extract` with a
`vector.extractelement` op). This patch is bailing out folding when 0-D
vectors are found.

Reviewed By: nicolasvasilache, hanchung

Differential Revision: https://reviews.llvm.org/D151847