[mlir][Standard] Extend n-D vector lowering to LLVM to [s|z]exti ops.

[s|z]exti ops do not have the same operand and result type.
As a consequence, the lowering of the n-D vector form needs to be relaxed a bit.
This revision additionally performs a few NFC renamings of variables to make them more intuitive.

Differential Revision: https://reviews.llvm.org/D95760